The Phalanx Covenant: Part 2 - S5-E2
Revealing mistake: The shot of Cameron Hodge glaring at Xavier after Xavier asks him about his lack of humanity is a recycled shot from the previous episode. (00:14:30)

The Phalanx Covenant: Part 2 - S5-E2
Revealing mistake: When Forge says "That's where we'll begin our assault," his mustache disappears from his face. (00:14:50)
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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