Cameos, in-jokes, re-used animation and other trivia from Monsters, Inc..
- The Pizza Planet truck is parked next to the trailer from A Bug's Life, where Randall gets banished at the end.
- Roly Poly Clown appears at the end.
- Boo has a Pixar ball, a Nemo toy and a Jessie doll in her room.
- Rex appears in the outtakes.
- Randall turns the same color as Andy's bedroom paint job from Toy Story at one point.
- The wall behind the Octopus in Harryhausen's has Marlin from Finding Nemo on it.
- Nemo also appears on the door of the Trailer Son and Mom when Sulley throws Randall into it.
- The toy plane from Toy Story appeared on the top left of the shelf in the little boy's bedroom at the beginning.
- Sulley has 2,320,413 individual strands of hair. 25,336 of them are "key" hairs, which are used to guide the motion and shape of the other 2,295,077 hairs. A technical director hand animated the hairs on his fingers to make them react to Boo's touch near the end of the movie.
- The letters "FIZT" on the door machine come from an application developed to control the movement of Sulley's fur based on his actions and other factors such as wind.
- The name for Mike Wazowski came from the father of the Muppet performer Frank Oz.
- In one of the children's bedrooms at the end of the film, posters for Disneyland's Sailing Ship Columbia and Tomorrowland are seen.
- Randall Boggs, voiced by Steve Buscemi, at one point says to Fungus, "If I don't see a new door in my station in five seconds, I will personally put you THROUGH THE SHREDDER!" Buscemi starred in Fargo, a movie in which his character gets put through a wood chipper/shredder by his murderous accomplice.
- On the top scarers leader board, under Sulley and Randall, is a monster named Ranft. This was probably to Joe Ranft, who is one of PIXAR's storyboard artists. He also did a couple of voices in films.
- All of the Scarers' last names except for Sulley, Randall, George Sanderson and Ricky Plesuski are also those of actual Pixar staff members.
- The Hidden City Cafe is seen in the opening scene, which, according to a teaser trailer for WALL•E, is where they thought up the idea for Monsters, Inc.
- It is also the only film in which the A113 gag is nowhere to be seen, although "A13" can be seen on the walls inside the Door Vault near the end of the film, making it an "A" and one "13".
- The scene where Sulley thinks that Boo has been crushed to death in a trash compactor references the Looney Tunes short Feed the Kitty, right down to Sulley even mimicking Marc Anthony the bulldog's reactions to "Boo"/"Pussyfoot" being "shredded alive" by the compactor/"cooked alive" in an oven and subsequently lamenting her "death" after retrieving a garbage cube containing one of Boo's costume's "eyes"/cookie shaped like a cat.
- The preschool teacher who brings her class to the Monsters, Inc. factory during Boo's escapades inside the factory and blending into said class is named Ms. Nesbit, after a throwaway gag in Toy Story in which Hannah Phillips assigns that name to Buzz Lightyear after Buzz attempts to fly out of a window after learning the truth about him not being a flying toy after watching a TV commercial about his toy brand only to fall off the balcony and crashing onto the first floor and severing his left arm, before being rescued by Hannah and turned into one of her "dolls."
- This is the first Pixar film that was not directed by John Lasseter.
- It is also the first Pixar film of the 21st century.
- This is the last Pixar film to have outtakes at the end of the film.
- The Blu-ray re-release version of the film's closing credits lacked the outtakes as in the original theatrical and home releases as such has been relegated to a bonus feature, instead it features stylized monster body parts (like in the opening credits) and closet doors surrounding the credits.
- All of the digital displays (such as the clock radio, the "Days since the last accident" counter, etc), are all made up of Nixie tubes. Nixie tubes were digital displays made up of digits formed from light bulbs with wiring shaped like letters or numbers, much like neon signs.
- This is Pixar's first attempt at animating fur.
- It's also Pixar's first attempt at animating loose clothing like skirts and dresses (in this case, Boo's nightshirt). In the first two Toy Story films, all human girls and women, (Ms. Davis, Hannah Phillips) wore pants since it was very difficult at the time to animate skirts and dresses, Bo Peep's dress included a hoop skirt, and the various Barbie dolls were all seen wearing tight clothing. The second instance of Pixar animating loose clothing was in Finding Nemo with a woman's skirt in the waiting room (most noticeable in the original 2003 US home release-exclusive fullscreen version) toward the end of that film.
- This is the only Pixar film whose original VHS and DVD releases lacked a paperboard slipcover.
- There are at least 100 different monster "races" seen in the film.
- Due to software limitations, the artists were mostly limited to 1 hairy character per shot.
- This film was nominated for the first Academy Award for Best Animated Feature in 2001, but lost to a DreamWorks film named Shrek. This makes Pixar one of the first three animation studios to be nominated for that category.
- This is the first Pixar film to mute the beginning of the Disney and Pixar opening, followed by The Incredibles, Ratatouille and Inside Out.
- Before Billy Crystal got the role of Mike, he was given the option to voice Buzz Lightyear for Toy Story but decided not to. After he saw the film, he said that he made a big mistake. Pixar said that if anything were coming up that they will consider him. When he was given the option to do this film, he immediately said yes.
- This was the only Pixar movie that had a blue VHS tape instead of black.
- The code 2319 stands for White Sock - W is the 23rd letter in the alphabet and S is the 19th letter.
- 90% of all the monsters in the film have Mike's tongue.
- The producers had a hard time getting Mary Gibbs, Boo's voice actress, to get her voice recorded because they couldn't get her to stand still at the microphone. They ended up carrying a microphone around her when she played the character.