Continuity mistake: When they go to the Scare Floor to put Boo back in her door, Mike swipes the card key and the numbers "240492" appear on screen. In the next shot, the numbers on screen are now "43496". (00:39:10)
Continuity mistake: When Sulley believes Boo is in the trash compactor, a cube of garbage comes out. In the first shot of the cube, the light used to make the "eye" is seen sticking out. Then as the conveyor belt stops, the light is now laid down on the cube. (00:44:20)
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)
Other mistake: In the door vault scene, Boo's laughter causes all the red lights on the doors to generate and fully power. Unlike the other two scenes - back at the apartment and the trash compactor - this resulted in a blackout. (01:09:09)
Suggested correction: In the house and at the compactor, it caused an overload. This resulted in a blackout. The doors seem to work differently.
Other mistake: Near the end of the film, when the identity of 0001 of the CDA is revealed, number 0112 is shown both standing next to 0001 as Roz enters the room and behind Sully and Mike on the opposite side of the room. (01:16:45)
Factual error: After Mike rebuilds Boo's door, it shows a few crack lines but is otherwise intact. But the shredder created a lot of sawdust so it would be impossible to rebuild the door using only the wood pieces. (01:20:00 - 01:24:10)
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 is bringing Sully to the fixed door he has no slivers or band-aids on his hands, then when he says "it was a lot of wood to go through" you see he does. (01:24:10)
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, when the simulation is terminated, as Phlegm pulls a jack off his back, the large checkered rug at the foot of the bed is gone. It should be seen on the floor in front of him, in this shot facing the wall as it goes up.
Continuity mistake: When Boo is in Mike and Sully's house, there's a "jazz" poster on the wall near the door, but when Boo is crying, it's gone.
Other mistake: In the scaring simulation room, the alphabet wallpaper border at the top of the wall has the "J" written backwards.
Suggested correction: The walls of the room also have nonsensical drawings like a giraffe with two heads. The implication is that the monsters designing the room didn't entirely know what kids would actually have in rooms (like they didn't know that giraffes don't actually have two heads). So they thought that humans have the J backwards.
Whether or not the backwards J was intentional or as a joke, it would still seem unreasonable for the monsters to get it wrong when you see they got the pictures for each letter seen correct "jaguar", "kangaroo "pig", "quail", "rhino." I would at the very least call it a character mistake.
As the original correction said, it's just an example of the monsters not understanding the human world. Not a mistake.
Yet they know the alphabet? I doubt it.
Continuity mistake: After Boo falls asleep, when Sully says, "Hey Mike this might sound strange, but I don't think that kid's dangerous," Mike adjusts the lamp on the desk making the right bulb lower than the left. Next shot, as Mike turns, the right bulb is now higher than the left.
Factual error: In the scene where Waternoose and Sully are talking, Waternoose gets himself some "coffee", which is sludgy and thick and takes quite a while to pour from the machine. Seconds later, he takes a very quick sip. Since the liquid was so thick and there was little in the cup, it would be impossible for it to pour that fast.
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: 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: 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.
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: 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.
