It wasn't on purpose. If John thought very carefully, then he could tell you for certain that it hadn't been on purpose.

But then the ad sidebar—those annoying ones that flashed and blinked at you—had shown him a costume shop. Not far from their flat. John could go after work. While he was on break from work.

So he did.

And it was displayed prominently atop some even more elaborate coat than Sherlock currently wore, and well. John was a bit sunk. He could afford it this year. And... as he nibbled his lip, the shop owner sidled up with a grin and said, "You're going to buy this one."

"Yeah," John agreed reluctantly. "I really am." And because Sherlock was an incorrigible snoop, he kept the giant hat box in his office at the clinic and waited until Christmas morning to fetch it while Sherlock was still dead to the world to bring it home and wrap it. He snuck some oddly-shaped objects (a mug, a razorblade box, a medication bottle) beneath the wrapping to keep him from guessing immediately. But why would Sherlock ever guess? There was no way John would—should—know. He grinned and set the giant thing upon Sherlock's sofa and then made tea, waiting until the man roused himself.

He ended up waiting until almost half eleven, but considered it worth the reward of seeing surprise fly across his flatmate's face.

"John!"

"Good morning, Sherlock. Ah-ah! Breakfast—well. Lunch first."

Grumbling, he slouched into the kitchen and stuffed a piece of toast into his mouth. "What on earth did you get me, John?" He crept closer, circling the giant wrapped monstrosity, going so far as to crawl across the back of the sofa.

"You can't guess?"

He sniffed. "I never guess."

John felt inordinately proud of himself and then snickered. "Go on then."

And the switch had been flipped. Sherlock tore into the paper with abandon, snorting at the found objects. "A hat box? You didn't...hat-man..." Sherlock frowned.

John shrugged and set his paper aside. "Open it."

Sherlock lifted the hat box lid off carefully. Then his eyes widened, brows shooting up, cheeks flushing with pleasure before it all shut down into a careful blank facade. "Who told you?"

John grinned. "Mycroft may have mentioned it a year ago."

Sherlock scowled.

"Put it on."

"Mycroft?"

"Put it on!" John leaned forward.

Another beat and then Sherlock was grinning and and lifted the grandiose pirate hat onto his head. He shuddered when he lifted his hands away and the full weight settled onto his riotous curls.

"Looks fantastic!"

Sherlock bolted to the mirror. "Yes. Yes it rather does." He turned, this way and that, admiring himself.

"Yes, you look lovely. Come and sit, you berk," John chuckled.

Sherlock settled, pleased, cocky. "I'll—"

"You're not wearing it to any crime scenes. Ever," John cut him off. "Not while I'm with you. But feel free to wear it any and every time we're in public with your brother."

Sherlock's wide grin was enough to ensure that it was a very good Christmas indeed.