Pixar showed more of Sully and Mike's relationship in Monsters University, although the prequel also retconned their origin that Monsters, Inc. established. Pixar turned another one of their original ideas into a franchise in 2013 with the release of Monsters University. The movie brought Mike Wazowski and James P. Sullivan back to explore their college experiences as they trained together to become scarers. Monsters University had plenty of connections to Monsters, Inc. as a result, but not everything lined up perfectly.
The main story of Monsters University revolves around Mike and Sulley's first semester enrolled in Monsters University's Scaring program. They both fail to get into the university's top fraternity, which initially puts their scaring futures in jeopardy. However, the rest of Monsters University shows Mike, Sulley, and some other misfits at the college train for a scaring competition that, if they win, will gain them acceptance into the Scaring program. It gives the prequel movie a chance to really establish how and why Mike and Sulley become the best friends audiences first met in 2001.
Related: Why Monsters At Work Is Better Than A Third Movie
The issue with Monsters University telling this story, though, was that it meant retconning Mike and Sulley's origin. The issue arises as a result of the prequel movie making college the first time that Mike and Sulley met. It was revealed in Monsters, Inc. that they've been friends since the fourth grade. Mike mentions this briefly and says that he and Sulley have been best friends ever since. However, Monsters University clearly retcons that fact. They are complete strangers during the prequel movie and clearly haven't had a prior friendship.
The retcon to Mike
Read more on screenrant.com