The Phalanx Covenant: Part 2 - S5-E2
Other mistake: When Mr. Sinister, Beast, Forge, Warlock and Amelia are walking towards Magneto, they walk in place for several strides. (00:08:30)
The Phalanx Covenant: Part 2 - S5-E2
Other mistake: When Forge says "Now we need offensive power," he opens up a containment bubble with no one inside it. The bubbles are transparent, so he would have known whether or not someone was inside it and it would make no sense for him to open up an empty bubble since he was in need of reinforcements. (00:15:55)
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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