Deliberate mistake: Grug's loin cloth gets ripped by a lion at one point. The rip magically fixes itself just moments later. (00:53:15)
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.]
Deliberate mistake: When Lex busts Joker out of Arkham Asylum, he uses his Deconstructor gun on the LEGO wall. It takes it about 8 seconds to pull the LEGO apart for Joker to get out. The same amount of time is taken when he demonstrates it to Joker and frees Penguin. Yet when Joker grabs it and uses it to free Catwoman and Two-Face, that time is cut in half. And then yet again when they are leaving on the chopper, Joker fires it 3 more times, freeing Poison Ivy, Harley Quin, and The Riddler. It barely takes a single second on each of those. (00:17:40 - 00:19:35)