When all the monsters are entering the Scare Floor near the beginning, the monster that is supposed to have no eyes has no eyes, then it has 3 eyes when walking, then back to none after it gets to the desk where it is assigned. These are two separate monsters, as can be seen in slow motion when the camera pans up to the top of the scoreboard. The monster with no eyes is seen near the bottom, whereas the one who looks similar and has three eyes is near the top, right under Randall.
When Mike and Sulley enter the Scare Floor the morning they sneak Boo into the factory, the clock says 8:50. Barely a few minutes pass before Mike is chased by Randall. They stop at a clock which then says 11:55.
When Sulley runs into the locker room to shove the toys from Boo's room into a locker, he is seen putting them into locker #193 then slamming the door with both hands. When the camera angle changes, he removes his hands from locker #190 even though his hands never moved from the locker between shots.
In the international version of the video/DVD several instances of written English text have been replaced with universal symbols (see Alternate Versions). However on several occasions they switch back to the original appearance. For instance: The "Standby/Scare" sign in the factory, the "Contamination alert" on the video wall during the first 2319 and the joke on the video wall in the final scene.
When Mike walks away after congratulating Celia (in the beginning of the film) he passes a marking in the floor. In the next shot, viewed from above, he is much nearer her than in the shot before, which can be seen by the floor markings.
When Waternoose congratulates Sulley on an 'impressive display' of scaring, different shaped monsters are just coming in to shot to walk past behind Sulley. The shot cuts to Waternoose and then back to Sulley in just a couple of seconds and the monsters behind him have gone.
On the Scare Record screen, where it lists all of the scareers, all of the monsters are listed by their last names (exp. Sullivan for Sulley while his first name is James). But, one, and only one, is listed by his first name, Randall Boggs.
In some scenes, Boo's giggles and screams do not produce any power spikes (e.g. her scream right before Sulley rescues her from Randall's scream machine and her giggles when playing hide-and-go-seek with Sulley in the bathroom).
The standard-issue energy canisters for collecting screams - as shown early in the film when Waternoose is addressing the new recruits - are clearly outweighed by the far bigger laugh-collecting canisters implemented by the end of the story, however close-up shot of both show that they are marked as having precisely the same capacity: "8400 SUV"
The goof items below may give away important plot points.
At the end of the film, after Mike got his first customer when he burped out the microphone, Sulley comes to talk to Mike and the "Think Funny" banners are still there. After Celia calls Mike "Oh Googly Bear" at the next shot, the banner that was above Mike's head has gone.
When Mike and Sulley are running through closets around the world, the times of day do not line up. Daylight is shown in Japan, darkness in Nepal (3 hours behind Japan), and daylight in Paris (7 hours behind Japan). While it's possible it could be very early morning in Japan, overnight in Nepal, and very late summer daylight in Paris, then it would have to be mid-afternoon on the U.S. Gulf Coast where Randall ends up - not the evening that is shown.
Towards the end of the film, when Mike and Sulley meet the CDA "number 00001", at least three CDA monsters have the same number, 00112 - the same texture had been used on several models to reduce modeling time.