Revealing mistake: Junior can't get on the Crazy Dance because he is a milimeter too short. After seeing Murph (the 20-year old in his class) and Trixie (on heels) get on it, Junior speeds up the ride and all the kids on it get dizzy and barf everywhere. If you look where the puke shoots out, it's a bit obvious it's not coming from their mouths, but from a point behind them. Most obvious are Trixie and the boy wearing glasses.
Revealing mistake: The vomit scene was probably filmed in front of a green screen. When the boy with glasses vomits, he wears green pants and we can see through the speed of the ride. [Visible on full-screen].
Revealing mistake: In the ending of the vomit scene, a man and a woman vomit. But the woman's mouth is closed for a second while the vomit machine is on.
Answer: She didn't. Junior deliberately contaminated her blood sample with the rabid dog blood he swiped from the vets.
Brian Katcher