This actually happened in second grade. It was hotdogs for lunch at school, which were my favorite food by far. My friend knew this and bet me I couldn't eat 4 hotdogs, so I said let's just have a competition. Close to five hotdogs in I conceded the match. Not bad for a 7 year old, but my skinny little friend ended up eating like 6 of them.
So I'm not exactly feeling top notch after all those dogs and then halfway through our next class there's a knock on the door and it's my mom. I'm confused and then it dawns on me that she had told me this morning that I had a dentist appointment in the afternoon (I was a spacey kid and had forgotten).
We drive to the dentist and the motion of the car really wasn't sitting well with me but I figure I'll be fine once we arrive. Wait for a bit in the lobby and that classic dentist office smell really isn't helping anything in the nausea department. Finally my name is called so I hop in the chair, get through the pleasantries, am starting to feel a bit better and think it'll be a breeze. Dentist puts on his latex glove, I open my mouth, and he's barely gets his finger not even an inch in my mouth when it all falls apart.
I watch in slow motion horror as I feel my gag reflex engage, see the dentist's eyes shift to quizzical concern and then wide-eyed "oh no" shock as I projectile vomit an entire stomach full of hotdogs everywhere. Ev. Ry. Where. The room looked like a hotdog vomit grenade went off and no one was spared. I distinctly remember vomit dripping from the dentist's mask and my only thought being "so that's what those are for." Everyone froze, completely silent, all of us mortified and no one having a clue what to do. Finally the dentist shakes his head, calmly turns around and leaves the room. My mom grabbed me and took me straight home, not a word. I never saw that office again.
Part of me thinks this doesn't belong in TIFU since hotdogs are awesome and dentists and school sucks and I got a ton of the former and none of the latter out of that day. I'll let you guys decide though.
TL;DR: Ate as many hotdogs as I could and then projectile vomited them in my dentist's face