Continuity mistake: When Sulley places the menu on the table he puts it face down (logo first), then when he lifts up the table, to quickly get Boo, the menu is face up. (00:26:05)
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)
Deliberate mistake: When Boo laughs repeatedly while playing hide-n-seek with Sully in the men's room, nothing happens to the electricity - no surges as with all other times she laughs. [In the audio commentary by the directors, they explained that it would've been distracting if an energy surge occurred every time Boo laughed, so they simply didn't address the issue. A reasonable course of action, but still a mistake.] (00:37:30)
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: 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 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.
Other mistake: When Sulley and Mike take Boo into the Factory, they see all of the CDA agents. There are 3 talking to Mr. Waternoose(as we see seconds later). One of them is number 00002. When they are done, they escort Mr. Waternoose to somewhere else. However at that very second another agent comes in with a metal detector thing. His number is also 00002. (00:35:20 - 00:36:10)
Continuity mistake: In the opening scene, on the floor there is a toy train on circular tracks when the monster sneaks in. When the monster trips backwards over the ball and skateboard then falls onto the jacks, the train and tracks are gone. (00:01:49)
Other mistake: In the scene where Mike and Sully walk in on the CDA overrunning the power company, there is a CDA agent with number 00112 behind the main desk, and another with the same number talking to the boss a second later. (00:35:10)
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)
Audio problem: When we see Mike during the slumber party, we hear seven bells ring for the seven filled cylinders, but there are only six cylinder-filling noises. (00:16:50)
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 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.
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: 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: 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.
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.
Suggested correction: Sully may have unplugged the alarm clock and put it somewhere else, especially when there's a "dangerous" child in his home, and he wouldn't want his things to get contaminated.
This correction is a stretch. If he doesn't want his things contaminated, why not remove the pictures and lamps? Why let her sleep in his bed with his blankets?