Night of the Sentinels: Part 1 - S1-E1
Deliberate mistake: When the Sentinel is first walking through the mall, its footsteps are so heavy that it shakes the building. When it is shown walking past the shop Gambit is in, there is no shaking of the building, presumably done so Gambit wouldn't notice it. (00:04:08)
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)