Continuity mistake: When Mike and Sully are in the ice cave, and Mike is starting to throw the a Snowball before Sully says, "None of it matters anymore" the snowball turns from yellow to white for a few seconds, and then back to yellow.
Continuity mistake: During the scene on the scare floor, there is a world map above the entrance. To the right of the map are two signs that read STAND BY and SCARE. In some shots, the signs change from written words, to symbols of a monster standing still (STAND BY) and roaring (SCARE). This only occurs in the international version.
Factual error: The map of the world on the scare floor shows Australia without Tasmania. This is best seen late in the movie after Boo has returned home, Sully is now management and when Mike asks him to follow him you can clearly see the world map behind Sully showing Australia and no Tasmania. (01:23:30)
Continuity mistake: When Mike, Sulley and Boo are walking to the Scare Floor after leaving the locker room, Mike's feet and legs are soaking wet. His feet are suddenly dry by the time they reach the Scare Floor. (00:38:45)
Audio problem: When Mike and Sulley walk through Monstropolis, Mike goes to his car. An automatic unlocking sound from the car, but the keys Mike is shown holding appear to be the keys one would use to manually unlock the door. (00:07:20)
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.