Continuity mistake: After Mike sprays his eye with the disinfectant it goes red, but just after he snatches the doll from Boo, his eye is fine again, and then goes back to being red for the rest of the scene.
Continuity mistake: The scare chart shows a picture of the scarer and the number totals. In the shot that shows Mike repeatedly taking out tanks whilst Sulley is scaring the child, the names of the people have suddenly appeared next to the pictures of the scarers that weren't even there in previous or following shots. This only happens in the international version.
Continuity mistake: Near the end of the movie when Waternoose "calls" for Boo's door along with the CDA, it arrives into the factory on the "outgoing" door track.
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?
Continuity mistake: In the scene where Mike and Sully are in the locker room after a day of scaring, discussing plans for the evening, George Sanderson's assistant is seen in the lower left side of the screen, having difficulties removing his helmet. In the scene immediately after, Mike and Sully are walking down a large hallway that leads to the Monsters Inc. main lobby. George's assistant is seen in the background in the hallway, wearing his helmet. Even in the shot where Roz is inquiring about Mike's paperwork in the lobby, the assistant is still seen crossing the lobby in the background, wearing his helmet.
Other mistake: The scare leaderboard shows scarers by their surnames, except Randall, who is shown by his first name only.
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."
Continuity mistake: When Sully and Mike are practising, their chairs are switched. Sully's is on the right Mike's is the left. When they sit to watch the commercial, they are switched back.
Character mistake: When Mike pulls out the first Scare Report, the word "yield" is spelled wrong.
Continuity mistake: Two people put an employee of the month picture of Mike and Sulley on the wall. The photo is noticeably crooked. In the shots of the two people talking to Mike and Sulley, the photo is correctly positioned without being touched.
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.
Answer: The Doctor says to the patient, "What seems to be the problem?" The patient lifts his arm and says "It hurts when I do this." The Doctor says "don't do that."