Just another middle-aged, overweight, straight female who loves reading (sometimes homoerotic) stories about YA-lit characters. My few humble offerings are on AO3 under Fyrebird, because that's where they were moved to when HP Fandom shut down.

Common Misconceptions in Harry Potter Fandom

Your Hogwarts letter arrives on your eleventh birthday.
This would be phenomenally stupid. The school doesn't have a correspondence secretary that keeps track of birthdays so they can send out greetings. Instead, like every other school in the world, the letters are sent out in the summer, for the upcoming year. Harry only received his letter on his birthday after at least a week of failed attempts. Harry's letter gives July 31 (his birthday) as the deadline for replies, so clearly they did not expect him to receive it on that date. So no, Hermione hasn't had almost a full year to read and memorize her school books: she found out in July, just like every other student raised in the Muggle world.

First Years are not allowed on quidditch teams.
Not a rule. First years are not allowed to have their personal brooms at school (a rule that was waived for Harry because Professor McGonagall is so quidditch-mad that she forced him onto the team without even asking him, and bought the broom herself*—her being the source is only implied in the movie, but is stated outright in the book). Obviously, not having a personal broom is a severe handicap for one with sports aspirations, as the school brooms are of dubious quality, and thus it is unlikely that a First Year would make the team, but not forbidden.
*I've seen some fanfics have Harry get stroppy when he finds out she purchased it with HIS money, without asking him, on Dumbledore's okay.

Susan Bones is an orphan who lives with her aunt.
This is not stated anywhere in canon. It says that she lost her grandparents, uncle, aunt, and cousins, who all had the "honor" of being killed directly by Voldie. When (spoiler alert!) her aunt Amelia Bones is killed in book 6, it specifically says she lived alone.

Nymphadora "Say my first name and die" Tonks was still a student in Harry's first year.
Harry meets her after his fourth year, and she has been a qualified auror for a year. Auror qualification is a 3-year course (and hers might have even taken a bit longer because she had trouble with the stealth training). That means she left Hogwarts the summer before he got there.

Harry went straight to Hogwarts after shopping in Diagon Alley.
This is a movie error. Hagrid arrives at the hut at midnight of July 31, just as Harry is wishing himself a happy birthday. After sunrise (still July 31), he takes Harry shopping. At the end of the day, Hagrid takes him to the train station to go back to the Dursleys' house for the month of August. Harry has to get his Uncle to take him to King's Cross on September 1. Granted, in the books this was covered by a few paragraphs at the beginning of chapter 6. ("Harry's last month with the Dursleys wasn't fun.") The movie has Hagrid just drop him off to take the Express straight from his school shopping. What a whirlwind that would have been, to go off to magical boarding school the day after finding out about it!

Hermione was always steadfast, and never failed Harry.
Wrong! Again, this is a book-versus-movie issue. When Sirius anonymously sent him a broomstick for Christmas (which the movie changed to the end of the school year), Hermione was so sure that it was booby-trapped that she ratted him out to McGonagall, who took it away to be completely stripped down, and didn't return it for over a month. Coincidentally, quidditch-mad McGonagall (that should be her nickname) gives it back right before Gryffindor's next game... in February (the book isn't clear on when in February, so it could be closer to two months for all we know). Imagine that you're a mistreated orphan who never got any gifts until you started school, and your so-called friend has your best-ever gift taken away and completely dismantled before you get a chance to use it even once. As an adult, of course you applaud her for worrying so much about his safety, but to a contemporary she's a rotten tattle-tale. Don't get me wrong, I like canon Hermione, but I also enjoy a good Hermione-bashing fic because she's such a self-righteous prig at times. (I mean, come on: she makes up a study schedule for them! And badgers them to stick to it!) Not to mention that she got all bent out of shape that using the Half-Blood Prince's book was "cheating." Where was this morality when she got extra study time for an entire year by using a time-turner? She didn't use it just for classes, but for homework as well (or she wouldn't have been so exhausted). Could it be it was only because he was getting too close to her own grades? He did, after all, beat her to a prize by making a better potion.

Hogwarts school fees.
Hogwarts is a state-owned school, funded by the Ministry of Magic. There are no tuition fees, although students must provide their own uniforms, books, and supplies, as listed in the Hogwarts invitation. Harry's vault of money will easily see him through finishing school and getting set up, although it's unclear whether it's enough that he wouldn't have to get a job. (JKR, who admits to being bad at maths, was not very detailed when setting up her Wizarding economy. At first, a galleon was said to be worth approximately 50 pounds—that first morning, the owl wants a measley 5 knuts to pay for the Daily Prophet, and seven galleons for a wand is such a steep price that the Weasleys can't pay it. Then the exchange rate was reduced to 5 pounds per galleon, which is ludicrously low. It makes the Weasleys positively destitute in Chamber of Secrets, when their vault contains only a handful of sickles and a single galleon, coming to less than ten pounds.)

Lily and Petunia hadn't communicated in years.
In the very first chapter, when the Wizarding World is busy celebrating the demise of Voldemort, Vernon Dursley is an unwilling witness to multiple instances of abnormal behavior. He hears reference to "their son Harry," and suspected it might be his nephew. If there had been no communication, he would have no way of knowing his nephew's name.

Pet peeves in Harry Potter fanfics

Please don't put in the Sorting Hat song from the first book. We've read the books. If you haven't written your own (please don't), you're just padding your word count.

Same for Harry's Hogwarts letter, book list, and O.W.L. results. If you haven't changed anything, we don't need to see the whole letter again.

Get the names right! For example, there is no apostrophe in Gringotts Bank, but there is in St. Mungo's. Neville's parents are in the Janus Thickey (not Thickney or Thackery) Ward. The judicial body is the Wizengamot. Quirrell has double R and L, McGonagall has only a doubled L. There is only one R in Voldemort. Typos and autocorrect happen, in which case Mr. Filch can become Mr. Finch, but I will abandon a fic with Zambini as a character. That goes double for using the Americanized "Little Whining" for the Dursley hometown. (I'm an American, but even as a child I read many books by British authors, set in British places, and it didn't harm me to learn alternate terminology. It won't hurt you, either.)

Ravenclaw's animal is an EAGLE (movies notwithstanding). Don't call them "Ravens" unless the Gryffindors are "Gryffs" (which is perfectly acceptable, although I'd go with 'Claws if we're abbreviating). If the Gryffindors are lions, then Ravenclaws are eagles.

Parseltongue is a language. A Parselmouth speaks that language. Don't mix them up, or use them interchangeably.

A hollow is a hole, niche, depression, or otherwise emptied space. A hallow is something sacred or blessed. Don't ask me why JKR decided to use two nearly-identical words for important story concepts.

Apparition is a ghost or mirage. Apparation is JKR's made-up word for a form of travel. Don't listen to your spell-checker on this one; it will lie to you.

The Statue of Secrecy. I visualize something like the lawyers' "Lady Justice," only she's got her finger in front of her lips, shushing everybody. Double- and triple-check your spelling on the STATUTE of Secrecy.

Don't give us sentences that are primarily a list of names (or a long list of anything, really). Break it up into a narrative.

Similarly, don't spell out the class schedule in one indigestible lump. If you need to write it down for your own notes, go for it. But I don't need to see, "Monday is Care of Magical Creatures and Transfiguration, followed by lunch, then double Herbology. Tuesday is...). Only tell us the class when your students are going there (or not), or putting off their homework because it's not until Thursday, or something like that. This is especially true if you've tried to make some sort of table, because we all know that the format won't carry over to the archive site.

I get bored with stories that go into detailed menus and food prep. I don't care if it has lemon zest or a caper sauce. I get it: you like fancy food. Get on with the story, already.

If you want to pair Harry with either Ginny or Hermione, you need to change their canon relationships to make it believable. I know Harry/Ginny is canon, but (sorry, JKR) they are a BAD MATCH. I won't go so far as to say she's just a fangirl, but she did have a "celebrity crush" on him at the age of 10, that she never seemed to get over. She could barely even TALK to him until one day she jumps him and kisses him, one week after breaking up with her previous boyfriend, and suddenly it's Twoo Wuv. I'm not opposed to the pairing per se, but let's face it: she was such a non-entity in the books that Harry only noticed her spastic behavior until they brew a love potion in class (my headcanon is that he smelled it being brewed at the Burrow, not that it was indicating he loves her). Then she says she knows he'll "never be happy" unless he's hunting Voldemort, and that perhaps that's why she likes him so much. If that doesn't make her a fangirl, it makes her extremely immature — but they're only 15 and 16, so that's to be expected, and who the heck expects kids that age to stay together forever? (Truly, does everybody in the UK meet their future spouses in what we call high school? Not sure whether that's sad or frightening.)

If you write a "Harry gets betrayed by the Light side and sent to Azkaban" fic, have the courage to make him angry and/or vengeful. I despise the ones where he spends years in Azkaban, and then almost immediately forgives everybody and goes tra-la-la-ing through the daisies with them in no time.

Don't label your POV switches. I despise fics with "Harry's POV," at the beginning of a section. It should be obvious from your writing whose POV it is: all you have to do is put the person's name somewhere in the first few sentences. How hard is that?

A revelation or twist at the end of a chapter is not a cliffhanger. A cliffhanger is when a dire situation is left unresolved; will your character escape/survive/etc. "I'm pregnant!" is not a cliffhanger. A surprise visitor (other than the villain) is not a cliffhanger. And for God's sake, unless you are actually under 16, don't refer to it as a "cliffie." Baby talk is annoying (which is why Bellatrix does it).

Fandom Guilty Pleasures

Wards. Amazingly, they're never mentioned in canon, but they're a staple of the magical fantasy genre. They do set "defensive enchantments" around the tent while on the Camping Trip from Hell, which is the same thing, as far as I'm concerned. Wards were (barely) added to the extended canon in the "Harry Potter: Wizards Unite" game: there's mention of a course on "Wilderness Wards." As for 4 Privet Drive, what fanon calls "blood wards," were only referred to in canon as a charm, performed by Dumbledore, that was "sealed" when Petunia first took him into the house and has to be recharged by Harry's presence every year. (Pretty underhanded of him; who would stop to read the letter before bringing the baby into the house on a cold November morning? And then it's too late, haha!)

Potter family vault. The Potters are an old family. Despite their cottage in Godric's Hollow being damaged, and then the Ministry claiming it for a shrine (did they pay for it? Canon doesn't say.), there's no reason to believe that money is all they left their child. We know that objects are also stored in vaults (i.e., the Lestrange vault in Deathly Hallows). If Harry's vault has only coins, then the rest of their things must be somewhere else.

Helpful goblins. In canon, goblins are famously neutral in Wizarding affairs, refusing to take sides, but they hate the Ministry. So while none of the goblins in canon are particularly helpful (and Griphook is a backstabbing fiend), all it takes is to come up with a reason why helping your character is a poke in the Ministry's eye.

Bashing Dumbledore. I loved the books, truly I did, but Dumbledore drove me crazy. He's not evil, but he's so full of hubris that he hoards knowledge and thinks his plan is the only way (not that he shares it with anybody), and thus some of the things he does have evil results. What kind of educator tells the students that they must make allowances for an abusive teacher? What kind of school is he running, when the very first night Harry is there, they are warned of "a most painful death" just for going into the wrong corridor? (Does he know children at all?!) And let's not forget, he gave Harry his own property (the invisibility cloak) as a gift. (Hey, I totally bogarted this thing that could have saved at least one of your parents, but since I couldn't find the other trinket after all this time I guess I'll let you have it so you can find that thing I want you to find, so when I nudge you to risk your life you'll know how to use it.) The Headmaster's office is behind a locked and guarded door, unavailable to the students. He never answers a question fully or clearly: it's all half-answers, double-speak, and "when you're older." When he finally gets around to "training" Harry to take on Voldie, what does he do? Shows him videos of Tom's childhood, so he can understand him. Thanks for nothing, Dumbledore. (As Emerald Ashes said in an author's note: "OOTP!Dumbledore is a terribly well-written *villain*.") Of all the BS Dumbledore spewed at Harry, I think the biggest lie is when he said it was because he loved Harry too much. You can keep that kind of love, thanks. Not to mention that the Order of the Phoenix is arguably a cult: Dumbledore makes all the decisions, and overrules or disregards any and all objections: everybody is expected to (and worse, DOES) follow whatever he says, whether or not (usually not) he explains why, because he just "knows" what's best.

Independent Harry. The sassier, the better. He's been bruised and downtrodden as long as he can remember. Suddenly, he finds he has fame and money. We call that "leverage," suckers! It's not like the Dursleys trained him to depend on the adults in his life, and the teachers haven't done much better.

Triwizard Tournament stories. I love it when Harry has (finally) had it, and uses the opportunity to give it back with both barrels. He may be forced to "compete," but they've effectively emancipated him AND expelled him from Hogwarts (he's competing for a different, albeit unnamed, school, after all). He doesn't have to attend class or take exams (that's canon!), and the Tournament loopholes have loopholes. Let the games begin!

Floriography (AKA "Language of Flowers"). Besides that I've always liked this, Wizarding society has held onto so many archaic tools and traditions that it's completely believable that this has endured. After all, Prof. Snape's first words to Harry can be translated as, "I bitterly regret your mother's death." Asphodel (a lily) = "my regrets follow you to the grave"; wormwood = absence and bitter sorrow. (Fun fact: asphodel was once thought to be a cure for snake bites.) As for the Evans sisters, Lily means beauty, elegance, and sweetness; Petunia means resentment and anger.

Time Travel/Do-Over. We've all wished we could go back and change what we did, hopefully with a better outcome. Bonus if they say, "forget causality, I'm changing everything!" Start those butterfly wings flapping.

Bound magic (that gets unbound). Usually a subset of Helpful goblins, but he could just go to a healer. Harry's hair regrowing might indicate a latent talent for metamorphmagery. I guess this is a bit of a cheat to give Harry talents he didn't have in canon — but if we wanted canon Harry, we'd just reread the books, now, wouldn't we?

Living in a wizarding trunk/tent. Whether Harry goes full independent, or just wants to mitigate the privation at the Dursleys' house, trunks and tents with huge space are canon; and he can take everything he owns, everywhere he goes. The first one of these I read, way back in the Stone Age, had Harry buying a whole set of trunks so each of his friends had an apartment. They all had active floos, so they constantly went back and forth visiting each other.

Secret bookworm Harry. Many bullied students take refuge in the school library. You'd have to at least pretend to read, lest the librarian evict you. Might as well read while you've got a book in front of you.

Hermione and/or Weasley bashing. It's not that I don't like them, it's that they're the most fleshed out characters, so we have a handle we can use to twist them just that little bit. (After all, nobody is ever accused of Daphne Greengrass bashing.) So, Ron becomes a feckless bottomless pit. Hermione is a bossy shrew. Ginny is a vindictive, manipulative harpy. Molly's meeting with Harry at King's Cross was orchestrated, to get the poor neglected child to "latch on" to a family of Dumbledore's choosing. Harry's feelings for Ginny came from a love potion. (This one is absolutely possible: Molly Weasley admits to using one to get Arthur's attention; Ginny loiters by the love potions in twins' shop, although she says she doesn't want one and the twins say they wouldn't sell her one; and Hermione thinks they are neither Dark nor dangerous, though she doesn't want just anybody using one on Harry.)

Rehabilitated Draco. Canon Draco is a little shit, but he didn't have to stay that way. I've always been annoyed that Dumbledore and—dare I say it? —JKR both wrote Draco off. Dumbledore even said something to the effect of Draco already being "lost" by the time he started school at ELEVEN. When clearly, Draco was simply the pampered child of overbearing parents who'd never had to think beyond what he'd been told. He was ALWAYS redeemable; but then, Dumbledore wasn't in the redemption business. Just ask Snape, who even with literally YEARS of dedicating his life to atonement, still had it held over his head constantly.

Rehabilitated Snape. This is only believable in the early years, definitely before fifth year, because after too long it would be completely unforgiveable. (Don't even get me started on Harry naming one of his children after TWO abusive teachers.) Yes, he's a nasty bully in Harry's first class. But a man can realize he's wrong, and take steps to mitigate a bad first impression. He arguably has to favor his house (at least in public), because the entire rest of the school is AGAINST his house on general principle. That his behavior encourages bullies is a common mistake, one that Dumbledore himself made with the Marauders. He's a bitter, friendless man, who has Old Twinkle-eyes grating on his last nerve on a regular basis, and definitely knows how to hold a grudge. However, he's also an intelligent, driven man, who was creating spells while still a student and became Potions Master and head of Slytherin house at the tender age of 21. We have only Sirius's extremely biased word that he was fully versed in Dark Arts as a first year (JKR was really fond of vilifying eleven-year-olds by proxy), and Dumbledore implied that he'd have been better off in another house once he'd had a chance to mellow a bit ("I sometimes think we sort too soon", DH Chapter 33). Based on what we later learned of Snape's early childhood, it's quite believable that the "ambition" that saw him sorted into the house of snakes was simply to be powerful enough to never be that vulnerable again — an ambition that the Marauders would have hardened into obsession.

As an additional note on both Draco and Snape: during her publicity tour for the Order of the Phoenix book, JKR admitted to being astonished to find out that either of them had fans. This, I believe, is why the very next book had both of them becoming significantly nastier. Clearly, we the readers were supposed to completely despise them, perhaps so that it would be more of a "surprise" that either of them were capable of the slightest positive behavior or motivation.