Other mistake: When Xavier asks Magneto how much further they have to reach the top of the cliff, Magneto tells him he would rather not know, suggesting they still have a long way to go. However, in the wideshot of the sky-riders flying in to attack the two, you can see Magneto is about three times the length of his own body from the top, and according to the book "X-Men: The Art and Making of the Animated Series," Magneto is 6 feet tall. This means he is only about 20 feet, give or take, from the top, which isn't all that far in terms of climbing a cliff. (00:02:44)
Other mistake: After Magneto says "Trust me, you'd rather not know," a hole appears in his pants around his right hamstring and then quickly disappears. (00:02:50)
Other mistake: Ka-Zar refers to Xavier by name despite only having just met him and Xavier never telling him his name. He even asked Xavier who he was only moments before. (00:09:19 - 00:10:17)
Other mistake: When Magneto is asking how the Savage Land mutates were able to infiltrate his citadel, Amphibious tells him it's because Mr. Sinister long ago found his secret passage. The room they are in leads to a balcony, and has no door sealing off the balcony. In a world of superpowered beings, some of whom can fly, it wouldn't take much effort to get into a building that isn't closed off. Finding a secret passage from the ground level would be unnecessary. (00:15:00)
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.
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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