When Ponyville is threatened with a missile filled with a powdered form of weapon-grade Poison-Joke, Rainbow Dash saves the day by hurling herself at the missile. The speed of her collision causes a sonic-rainboom, which destroys the missile and neutralizes the weapon-grade poison-joke powder...most of it anyways...

Rainbow Dash is immediately taken to a nearby hospital, where she is treated for a bad concussion. Unbeknownst to anyone, a few particles of Poison-Joke absorbed through Dashie's skull as she head-butted the missile into oblivion. Contamination of one's brain with poison joke often warps the way one sees the world, but nobody would've guessed that it'd change the way Rainbow Dash saw Fluttershy...

...and fewer than nobody would've guessed that it'd make Rainbow Dash believe Fluttershy was a tree...