Beyond Good and Evil: Part 4 - S4-E11
Other mistake: During the montage of people being erased from time, the exterior shot of the X-Mansion shows part of the mansion destroyed. The mansion is later shown to be destroyed by Graymalkin after the heroes defeat Apocalypse, but at this point in the episode there's no reason for the damage. (00:12:20)
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.
BaconIsMyBFF