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 Randall yanks Mike's arms upwards to represent the clock hands both pointing up, he pulls Mike's left arm as he says 'the big hand' and his right as he says 'the little hand'. However, as he says 'When the big hand points down' he bends Mike's right arm downwards.
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.
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.
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.
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.
When Mike, Sulley, Boo and Randall are going in and out of the doors in the door room, the time of day is inconsistent in the different parts of the world. At the same time, it is nighttime in Nepal and Florida (trailer park), and daytime in Paris, Tokyo and Hawaii (or the Caribbean).
In the opening sequence, a wide shot reveals that the soccer ball is under the bed, on the mat at the foot of the bed is a set of train tracks, and that one of the books on the toy box near the window overhangs the edge a little. On the shot where the camera cranes down under the bed, the soccer ball has now moved to be at the side of the bed rather than under it. Later in the sequence, Mr Vile steps back onto the soccer ball, which has now move to the foot of the bed rather than at the side, the train tracks have been replaced with a set of jacks/knucklebones, and the books on the toy box have moved slightly. In the next shot the books are back to the original position. Finally, in the video replay, we see the train tracks set up as they were originally.
In this movie, Mike says to Sully "You have been jealous of my looks since the 4th Grade" but this cannot be true as the sequel, Monsters University, is about Mike and Sully meeting for the first time at university.
In the door vault scene, the yellow door Mike, Sulley and Boo are riding on at first has the handle on the left (the reverse side of the door). Just before and during the rollercoaster-esque drop, the other side of the door is seen from the same angle, with the handle now on the right (as can be seen in the three shots). After the cable levels out, the door is round the correct way again.
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.
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).
In the commercial, we are told that Monsters Inc. matches children to the monster that is likely to scare them most. The monster in the beginning who demonstrated the 'dead door' said that his child was only six. Wouldn't they cut down on dead doors if they matched the door to a different monster who'd scare it more?
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.
During the chase through the doors at the end of the movie, the characters go through a door on the ground. None of the characters ever close the door, thus leaving the patch to the monster world wide open.
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.