As a prequel to Monsters, Inc. (2001), this is the first, and so far only time that Pixar has made a prequel film.
When Mike wakes up after sleeping on Sully's hand, Mike says "I know you're a princess and I'm just a stable boy." The line refers to one of Billy Crystal's previous screen credits as Miracle Max in The Princess Bride (1987) a story focusing on the love between a princess and a stable boy.
As they all enter the School of Scaring building, the students touch the front paw of the statue in front of the building. This is a reference to a tradition at Harvard University where students touch the left foot of the statue of John Harvard for good luck. As a result, the foot of the statue of Harvard is shiny and polished down. The animators included that onto the statue in the movie as well.
In the human summer camp, the child who comes out of the bathroom before Mike is about to leave roars at him. The roar is a sound recording used from Monsters, Inc. (2001) for the character Boo.
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