Mary Gibbs was so young that it proved difficult to get her to stand in the recording studio and act her lines. Instead, they simply followed her around with a microphone and cut Boo's lines together from the things she said while she played.

John Goodman pushed for Steve Buscemi to voice Randall.

It normally took 11 to 12 hours to render a single frame of Sulley because of his 2.3 million animated hair strands (total number of hairs: 2,320,413). Part of that animation process was done via the use of moving "attractor / repeller" particles that functioned like virtual magnets fans that would attract or repel the hairs proportionally over distance. By moving the particles along virtual paths they would provide a natural effect on the hair. An example of this could be seen in subsequent films such as the LOTR series, in which animated CG armies of thousands could be moved across a battle field, charging at orcs on the one hand and fleeing from trolls on the other.

In early drafts, the character of Boo was written to be six years old. The writers decided to make Boo younger because it would make her more dependent on Sulley.

John Goodman and Billy Crystal sometimes recorded their lines in the same room together, an unusual move for animated films, where actors more often work alone. Steve Buscemi and Frank Oz (Randall and his assistant Fungus) also recorded their lines together for the bathroom scene.

Continuity

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 Randall slithers through his door on the Scare Floor, after it's closed you see the handle is on the left side. When he emerges, the handle is on the right side.

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.



Factual errors

In the first scene Mike meets Roz where the latter confronts him for his failure to file paperwork, the sound of a chair squeaking can be heard at different points of this scene. However, Roz is a slug-like monster where it is not possible for her to sit on said furniture with this kind of body shape.

In the door warehouse during the chase when boo laughs looks like hundreds of doors become operational. You would think there is a very good chance at least ONE person would accidentally stumble into monster world.

During the first 2319 incident, one helicopter is flying directly towards the factory building. A street vehicle then obscures it, but based on the chopper's speed and trajectory it would have crashed into the factory, with no time to deviate.



Incorrectly regarded as goofs

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.

In some scenes, Boo's giggles and screams do not produce any noticeable power spikes (e.g. her scream right before Sully rescues her from Randall's scream machine and her giggles when playing hide-and-go-seek with Sully in the bathroom). However, as established in several scenes in the prequel Monsters University, mild amusement (such as that of the young campers Mike unsuccessfully attempted to scare, and Boo's self-suppressed giggles when Sully is playing hide-and-seek with her) is insufficient to produce significant energy spikes. As for Randall's scream extractor, it was specifically designed to gather power from its victims' screams, and could very well have gathered some from Boo; the audience is not shown any of the energy canisters attached to it during her one scream (which was cut short by Sullivan's arriving to rescue her), and as was established in the "dead door" scene much earlier in the movie, the power rapidly drains away from a canister when a child's screaming is insufficient to fill it.

In Sulley and Mike's apartment where Boo sleeps in Sulley's bed during the night, she shows him her drawing of what appears to be Randall; yet she doesn't encounter Randall or witness his existence until later in the movie when she is being abducted by him and Mr. Waternoose while Sulley and Mike are exiled into the Himalayas. Correction: Yes she does! There are actually a couple of instances in which she recognizes Randall between the drawing scene and Mike and Sulley's exile. Examples include her running to Sulley for protection as Randall approaches the bathroom, and when she reacts in fear watching Randall retrieve her door to look for her.

Sometimes when Boo laughs, the lights flicker, go out or pop around her, Mike and Sulley, but not always during the movie. However, according to the DVD commentary, the directors thought it'd be too distracting for the audience if that happened every time she laughed.

Intentional incongruity: Drawings that Boo makes in the monster world (without other humans around her) are signed with the name Mary (apparently in tribute to her voice actor) even though Boo/Mary is just starting to learn how to speak, never mind write.



Revealing mistakes

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"

When Mike is about to put his contact lens in, the lens does not reflect in his locker mirror. His hand reflects as if it is holding nothing. This is most noticeable in the Blu-ray version.

When Mike and Sulley are setting up Waternoose in the simulation, the J on the wallpaper is backwards in certain clips.

A close look at the newspaper which Fungus presents to Randall (reporting on the incident with Boo at the restaurant) shows it to be a prop fabricated for the story's reenactment: though the headlines and picture caption are real enough, the actual articles are all randomly generated "peas and carrots" word salad gibberish.

The daily newspaper - the Monstropolis Horn shows a photograph of Boo with her arms stretched out and calling out "Boo!" like the said scene of the prior incident. But the photographer from the actual scene of the incident pointed his camera away from her while in a panic and took a rather unintentional snapshot just right before the moment the picture in the newspaper shows.



Anachronisms

The scare dates for a kid named Albert Lozano in recent records range from February 8 - June 2, 1999 - being recorded in intervals of up to 1-3 weeks at the most. However, Monstropolis's daily newspaper - the Monstropolis Horn's recent reporting says "November 2, 2001"; just like the original wide release date of the movie itself.



Audio/visual unsynchronized

When Jerry starts counting down from seven, the audio of him saying "one" is missing.



Errors in geography

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.



Plot holes

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?

Celia makes a false announcement on Randall breaking the all-time scare record, to which the subject of the announcement responds by denying it. If Waternoose (the CEO at this point) found out on Celia's conduct in the workplace, she would have easily been fired; nonetheless she's still employed after Sulley takes over as CEO since Mr. Waternoose's arrest by the CDA and termination from his position.

Boo's monster disguise costume as told by Mike while he and Sulley sneak the human child into work is said to be made via chair fabric from Sulley's lounge chair, a couple lights that were from Mike's desk at home, and yarns from a mop. However, it is not explained what material they used to make the limbs.



Character error

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.

In this movie, Mike and Sulley didn't recognize the Abominable Snowman. But in the prequel, Monsters University, they met him in the Mail Room.

When Sullivan snatches the door key card from Boo's file, the file folder has her picture clipped into it, but the scare reports are the ones Mike was seen holding much earlier in the movie, which are for an eight-year-old boy by the name of Albert Lozano. Since the files and key cards for Albert Lozano and Boo (Mary Gibbs) were both on the desk at Mike's workstation at some point (and Mike was notorious for neglecting to turn in his paperwork), they most likely got mixed up when one of the villains (probably Randall) retrieved them from his workstation in order to cover up their having kidnapped Boo for their illegal experiments.

Number 5 is Mike and Sulley's station, while Randall's station is number 6. However, every time Randall calls down Boo's door, he calls it into Mike and Sulley's station, even after he tells Mike that Boo's door will be in his own station during lunch. Additionally, Mike tells Randall that Waxford works at station 6 (Randall's station), but Waxford is shown to actually work at station 9.

In the beginning of the movie, where Mike and Sully are leaving their apartment, Sully says to Mike that he's been jealous of his good looks since the fourth grade. Yet, they first met in college at Monsters University.