Deliberate mistake: In Monsters, Inc., when Mike and Sulley walked through Monstropolis to go to the company, Mike said to Sulley "You've been jealous of my good looks since the fourth grade". In this film, we're told Mike and Sulley met in college. [In the commentary, director Dan Scanlon explained that there was a lengthy scene that would have followed Mike and Sully throughout their years in elementary school, but it would have slowed the pace of the film and would have reduced the impact of their rivalry. Rather than spend a lot of time on a back story, Scanlon decided to have Mike and Sully meet in college, even though this was inaccurate.]Casual Person
Continuity mistake: At the start of the scene where OK arrives at the ROR house for the mid-game party, it is obviously dark inside except for flashing blue and purple lights, seen through the windows. However, when the OK gang enter the house, it isn't dark anymore and the blue and purple lights are gone.
Continuity mistake: When Mike and Sully move into their bedroom at the fraternity house, the door opens inwards. When they fall through the doorway, it opens outward.