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: In the carnival, Jr. gets upset because he was too short to go on a ride so he messes up the ride causing it to go faster. If you look in the background sometimes you get to see other people moving fast, revealing they put the film in fast motion here.