Continuity mistake: Waternoose grabs a scream can and starts releasing all the energy and the red line starts to go down. There are white lines over sections of the can. When he begins to release the energy, right at the very end of the first shot, the red line that goes down has not yet reached the first white line. In the very next shot, the red line has suddenly passed the white line.
Continuity mistake: When Sully and Mike are talking in front of the Abominable Snowman, and Mike starts throwing the snow cones at Sully, we see the snowman covering one of the snowcone holes with his thumb, while from a different angle it seems as though his thumb is between the two holes and the hole that he had just covered has an empty snowcone wrapper in it. (01:00:35)
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 picture of the scarer that weren't even there in previous or following shots.
Continuity mistake: When George comes out of the door with the kid's sock stuck to him the sock's toe points inward toward the middle of his back. When the CDA rushes in to remove it the sock toe faces outward. (00:17:40 - 00:18:25)
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.
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."