The Phalanx Covenant: Part 1 - S5-E1
Revealing mistake: The shot of police cars surrounding Sabretooth and him throwing one of them into the air is a recycled shot of the very first shot in the series from the season 1 episode "Night of the Sentinels, Part 1." It's even revealed a few minutes later in the episode that this isn't actually Sabretooth, but a Phalanx facsimile that has taken his form, which makes the fact that Sabretooth's actions are being recycled all the more dubious. (00:01:35)
The Phalanx Covenant: Part 1 - S5-E1
Revealing mistake: When Beast lunges away from the Phalanx disguised as several people in hazmat suits, after he lands he glides across the grass. (00:08:15)
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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