Other mistake: The scare leaderboard shows scarers by their surnames, except Randall, who is shown by his first name only.
Continuity mistake: When we first see Waternoose, he says a single touch of a child could kill them. A yellow guy throws his pencil and pad forwards out of his hands. When Waternoose walks over to the three people, the pencil and pad has moved to the right of the yellow guy without ever being touched.
Continuity mistake: When Mike is performing his standup routine in a boy's room near the end of the film, his eye quickly switches from green to blue, and then back to green right before he says "your room."
Plot hole: When Sully and Mike are talking in the toilets, during Boo and Sully's Hide and Seek game, Boo comes running in screaming and upset because she's just seen Randall, and he's not too far behind since all three can hear him talking offscreen to the CDA before entering the toilets, but how did Boo get to Sully faster than Randall without him seeing her, and how did Randall not hear her screams since they're all in the changing rooms which have an echo?