Revealing mistake: When Rogue's phone is ringing you can see that the phone is halfway absorbed into her mirror. (00:04:15)
Continuity mistake: The Colony's Acanti ship goes from having its face buried into the walls of the chasm and its body above ground to having its body partially buried in the valley floor and its face free from the wall. (00:06:50 - 00:08:10)
Revealing mistake: Wolverine's nipples literally disappear from his chest when he says "Do I feel lousy." (00:07:15)
Continuity mistake: Rogue's vanity has a teddy bear and a telephone on it. When Gambit comes into her room, the phone has disappeared. (00:03:55)
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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