9. Dan Scanlon - Monsters University
The Plot Hole: Pixar's prequel to Monsters Inc. depicts loveable monsters Mike Wazowski (Billy Crystal) and Sulley (John Goodman) meeting at Monsters University at the age of 18, but this directly contradicts the first movie, where Mike tells Sulley, "You've been jealous of my good looks since the fourth grade."
Director Dan Scanlon has openly discussed the issue, stating that they desperately wanted to exploit the monsters-at-Uni premise, enough that they felt fans would look past the continuity-derailing nature of the set-up.
He said, "It felt like the perfect place to do this, but we had that line. So we tried versions where they met young and then we skipped ahead to college. And we knew we didn't want to make Monsters Elementary...Pete Docter, the original director, and John Lasseter...finally said to me, 'It's great that you're honouring that, but you have to do what's right for the story.' So we made a tough decision to just have them be in college and put that line aside."
Though Monsters University didn't quite live up to the brilliance of the original, it was still a solid success, and few seemed to much care about the issue because the core premise was so irresistible. Scanlon did later offer up a half-baked explanation, though, that "since the fourth grade" was "an old monster expression". Hmm.