Continuity mistake: When the Nasty Boys and the Savage Land mutates confront the X-Men, you can see Slab is wearing the white belt that keeps his mutant powers intact. Later in the scene when he is attacking Cyclops, the belt is colored brown. (00:03:46 - 00:05:07)
Continuity mistake: When Slab tosses Cyclops, you can see Beast in the background, and he is crouching down. When the shot changes, he is standing upright. (00:05:09)
Continuity mistake: When Mr. Sinister is criticizing Xavier for fawning over humankind, Morph is barehanded. When the shot changes, he is wearing gloves. (00:11:34)
Continuity mistake: When Gambit is watching Rogue being taken away, you can see the bars of their holding cell are only about a foot taller than Gambit. Later in the scene when Wolverine is attempting to free the X-Men from the cell, the bars now go all the way up to the ceiling. (00:14:13 - 00:14:48)
Continuity mistake: When Barbarus throws Wolverine onto the control panel, there are numerous buttons on it. Moments later when Vertigo is using her powers to disorient Wolverine, some of the buttons have disappeared. (00:14:56)
Continuity mistake: When Shanna sees Kazar hiding around the corner, the guard has his eyes open. When the shot changes, his eyes are suddenly closed. (00:15:13)
Continuity mistake: When Kazar and Shanna hug after he has freed her and the other captives, the lock on the cell door is missing. (00:15:30)
Continuity mistake: When Kazar and his people are storming the citadel, Morph and Xavier are spaced closer together than they were a couple of shots prior. (00:15:32)
Continuity mistake: When Wolverine frees Morph and Xavier from their bounds, Morph and Xavier are suddenly closer to Magneto than previously shown. (00:15:32 - 00:15:59)
Continuity mistake: When Vertigo is telling Wolverine to become one of Mr. Sinister's slaves, the red diamond on Sinister's chest is the same blue color as his outfit. (00:15:32)
Continuity mistake: After Ruckus prevents Kazar and his people from entering the citadel, Brainchild pulls a lever next to the door to seal them out. The lever in question wasn't there when Kazar and his people were first shown outside the door. (00:15:44)
Continuity mistake: When the X-Men have their powers restored, Jean Grey is behind Cyclops on his right, near the front of the cell. When they begin running out of the cell, Jean is now several feet directly behind Cyclops, closer to the back of the cell. (00:16:18)
Continuity mistake: The glove on Magneto's right hand disappears after Mr. Sinister blasts him in the back. (00:17:20)
Continuity mistake: When Xavier helps Morph overcome Mr. Sinister's mind control, Wolverine, Storm, and Gambit are standing behind Morph, and Magneto is lying on the ground. When the shot changes, Storm, Gambit, and Magneto have disappeared. (00:18:35)
Other mistake: When Gambit is consoling Rogue just before she is taken to be tortured, he tells her that he loves her, and states that he has never said that to anyone before. But as we learned earlier in the season, Gambit was engaged in his youth to Bella Donna. Surely he told her he loved her at some point. Rogue is aware of this aspect of Gambit's past, so he would have nothing to gain by lying to her about it.
Continuity mistake: When the X-Men regain their mutant powers, Jean Grey is suddenly wearing her uniform instead of the casual outfit she was wearing when she was abducted. In the next shot and for the remainder of the episode, she is back to wearing her casual outfit.
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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