Continuity mistake: After the Scare Floor shuts down at 6:00 and Sulley goes back to do Mike's paperwork, the clock reads 5:48. After he returns again to put Boo back, it clearly shows 5:48. Finally, after Randall leaves and Sulley leaves the Scare Floor for the restaurant, the clock reads 6:48. (00:22:06)
Continuity mistake: At the beginning when you see the monster in the testing room from under the bed with the glowing eyes, you also see the soccer ball on the left side of the screen. This shot takes place on the side of the bed. After the monster gets scared, he backs up kicks the soccer ball, but it is now at the foot of the bed in the middle of the room. (00:02:35)
Continuity mistake: At the beginning of the film, we see Sully asleep in his bed and the alarm clock/radio on a bedside cabinet to the right. Later, when Boo climbs into Sully's bed, the clock/radio isn't there anymore. (00:04:50 - 00:32:30)
Continuity mistake: At the beginning when the monsters are at the testing room, you can see the boy in the bed seeing something opening the cabinet. He closes his eyes and opens them up again to see a sweater sticking out. But when another monster sees what he does wrong on the tape, the sweater isn't sticking out anymore. (00:03:40)
Continuity mistake: When Mike takes the toilet paper off his feet, it's lying on the floor in the hallway. But when his girlfriend comes and yells to him, it's gone.
Continuity mistake: In Sully's bed, Boo is afraid of the closet, and there are many drawings in the bed, but when she shows the drawing of Randall, the others are gone. They come back when she goes to sleep.
Continuity mistake: During the ending credits, they show Mike and Sulley redoing their "musical" number in front of all the monsters on the scream floor. During this Sulley blocks Boo out of the way by pushing her behind him. But when Sulley starts to dance, Boo has disappeared. (01:28:50)
Continuity mistake: At the start of the workday, Randall opens the first door with the knob on the left and hinges on the right. When he comes back out, the knob is on the right and the hinges are on the left. (00:14:45)
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).
Continuity mistake: When Randall catches Mike and tells him that he has half an hour to put Boo in her door he also says the door will be in his station. Later we see that the door is on station 5. But on the first day Mike and Sully are working on station 5 and Randall is on station 6.
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)Casual Person