Deliberate mistake: At Pete's diner, the owner puts the tray down, and instantly glasses appear in front of the 5 patrons. (00:02:50)
Deliberate mistake: There's at least two episodes (Season 1's "Captive Hearts" and Season 4's "Sanctuary, Part 1") in which a character shows video footage of the X-Men and various supporting characters in action, but the footage is merely clips from previous episodes.