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)
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: 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)
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)
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.
Trivia: Also in the end when Boo is handing her toys to Sully, besides Jessie, and the ball she hands him a clown fish. The fish is called Nemo, and it is from a new Disney and Pixar movie called "Finding Nemo", it is to be released in the summer of 2003.
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)
Trivia: Near the end of the film when Sullivan takes Boo back to her room and she is handing him her different toys, one of the toys she hands him is a Jessie doll. Jessie is the Cowgirl in Toy Story 2. If you look even closer you will notice that she also hands him the ball from "Luxo," one of Pixar's first animated shorts.
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 pictures of the scarers that weren't even there in previous or following shots. This only happens in the international version.
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)
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?
Bishop73