Days of Future Past: Part 2 - S1-E12
Deliberate mistake: When the crowd is protesting outside the Capitol, the same man with black hair and a brown outfit has been drawn numerous times in the shot. (00:10:23)
Deliberate mistake: The video footage Callisto has of the X-Men battling Sentinels at the Mutant Registration Center is just clips from that episode. (00:09:27)
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)
Answer: Chris Potter, the original Gambit voice actor, did indeed quit the role in the 4th season. His last episode was The Phalanx Covenant Part 1 so it seems reasonable that Fox would limit the use of the character even though it was recast. It doesn't appear that Potter left the role due to animosity, he stated in an interview that he wished to play Gambit in the first live action X-Men film.
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