In Monsters, Inc., about 7 minutes in, Mike says to Sully "You've been jealous of my good looks since the 4th grade, pal". Is this a figure of speech or did Mike and Sully know each other in 4th grade? Is the plot of Monsters University a contradiction to this statement?
This is discussed in the video below, in which Monsters University director Dan Scanlon explains that the line did negate the whole concept of the prequel, so they tried to explain it in a short scene where Mike and Sully meet when they're much younger. However, that scene proved problematic narratively, so they cut it and decided not to explain the goof and just make peace with it: