Other mistake: After helping Longshot in the alley, Jubilee tells Logan "Are you blind? He's on our side!" to which Logan responds "We don't know that." This dialogue is repeated verbatim when the Professor meets Longshot. In addition to being obviously re-used vocal lines, the context of Jubilee asking "Are you blind?" doesn't make much sense the second time.
No Mutant Is an Island - S5-E4
Plot hole: When Killgrave's bodyguard dumps Cyclops face down in the pool and leaves him for dead, it is night time. When Sarah is shown to have successfully resuscitated Cyclops, it is broad daylight. If Cyclops had been floating face down in the pool for several hours then he would surely be dead. No amount of CPR would have brought him back. (00:15:45)
Continuity mistake: When the camera pans around the team after Lilandra saves Xavier, Storm's communicators are colored white whereas they are normally red, and Rogue's gloves are missing below the wrist. (00:18:50)
Continuity mistake: When Jean Grey is stating how Magneto's powers intensified her own, the left side of her costume around her abdomen is colored yellow whereas it is usually blue (the right side remains blue in this shot). (00:13:30)
Continuity mistake: When Magneto is saying "You are the next step in man's evolution," the lower half of Sunfire's mask is flesh-colored. (00:11:55)
Audio problem: When Magneto is saying "I have heard your cries for justice and I now answer that call," his mouth is not moving. (00:11:45)
Continuity mistake: When Trish Tilby is giving the news report, she is first shown to be in front of a gray news van. When the shot changes and for the rest of the report, she is standing in front of a blue screen. (00:05:30 - 00:06:45)
Continuity mistake: When Cyclops, Wolverine and Gambit are watching Morph giving a statement on the news while masquerading as Xavier, Cyclops' hair looks like a bowl-cut rather than the usual semi-shaggy look he sports. (00:06:20)
Continuity mistake: When Moira is asking the X-Men how they intend on contacting Lilandra, Jean Grey's face is colored yellow. (00:05:10)
Audio problem: When Mr. Guthrie is saying "I worked with you inside the mines and shopped in your stores," his mouth is completely out of sync with the dialogue, sometimes not even moving at all. (00:14:45)
Other mistake: When Rogue rendezvous with Beast, Xavier and Gambit, she pinches Gambit's cheek with her bare fingers. This should have caused her to drain his powers and render him unconscious. (00:14:10)
Continuity mistake: In one shot before the nurse shaves a patch out of Gambit's hair, the neck portion of his black headgear is flesh-colored. (00:12:40)
Continuity mistake: The ice cream that Cannonball drops when Kenny confronts him disappears in subsequent shots. (00:10:50)
Continuity mistake: Kenny's hair goes from being light-brown to blonde. (00:05:50 - 00:11:00)
Continuity mistake: When Gambit is restrained in the chair, his gloves alternate between being his usual gloves that are fingerless at the thumb, index and pinkie fingers, to not being fingerless, to him not wearing any gloves at all depending on the shot. (00:10:05)
Continuity mistake: When Gambit jumps out of the ventilation shaft, there is no one around him. Then he is suddenly surrounded by guards who could not have converged on him so quickly unless they appeared out of thin air. (00:10:00)
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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