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)
Continuity mistake: When Kazar tackles Barbarus, Barbarus ends up lying on the ground between Kazar and Xavier and Magneto. When Xavier and Magneto begin running with Kazar, nothing about their stride indicates that they step over Barbarus, suggesting he is suddenly no longer in that spot. (00:08:50)
Continuity mistake: When Beast is theorizing that Xavier was coerced into contacting them, Gambit is sitting upright. When the shot changes, Gambit is now slouched in his chair (note his head is higher than the headrest in the first shot). (00:20:05)
Continuity mistake: When flashes of the different X-Men's faces are being shown while Xavier is being hypnotized by Sauron, Gambit's eyes are brown with white sclera, when they should be red with black sclera. (00:18:46)
Continuity mistake: When Sauron is hypnotizing Xavier, the space between Xavier and Magneto is greater than previously shown. (00:16:11 - 00:18:29)
Continuity mistake: When Magneto, Kazar, and Xavier are ambushed in the citadel, they are standing near the doorway leading to the stairs. When the shot changes, they are suddenly several yards into the room. (00:14:55)
Continuity mistake: When Kazar is taming the plesiosaur, its color changes from gray, to brown, and back to gray. (00:14:00)
Revealing mistake: When Wolverine notices Slab running off with Jean Grey, you can see the edge the animation cel moving across the screen from left to right. (00:12:34)
Continuity mistake: When Morph is performing his one man show, he is standing in front of a red and yellow stained-glass window. After he drinks the potion, the background behind him is now dark blue draping. It then becomes the stained-glass window again after he transforms into Mr. Hyde. (00:05:35 - 00:06:00)
Continuity mistake: When Amphibious is saying "Now we take them to the master," he is standing behind Lupo and Xavier. A couple of shots later, he is standing several feet away from them, next to a cliff wall. (00:04:45)
Continuity mistake: When Magneto is saying "Nicely played, Charles," the X emblem on Xavier's left shoulder is missing. (00:03:17)
Continuity mistake: In the wideshot of Magneto and Xavier scaling the cliff, Xavier is shown below and to the left of Magneto. When the shot changes, Xavier is now directly beneath Magneto. (00:02:25)
Continuity mistake: When Xavier is telling Kazar that Amphibious told him the Savage Land mutates were following a new master, his black shirt is suddenly the same color as his flesh. (00:09:47)
Continuity mistake: When Morph collapses to his knees in his dressing room, his yellow waistband is now white. (00:07:00)
Continuity mistake: When Beast is noting how odd Xavier's message was, the sclera of Gambit's eyes are white, when they should be black. (00:19:34)
Continuity mistake: When the Nasty Boys are walking Morph towards the wall to chain him up, the wall ends and leads outside on the left, and Xavier and Magneto are chained up only a few feet away on the right. A few shots later, the wall now appears to go on continuously, and Magneto and Xavier are considerably further away from Morph. (00:17:35)
Continuity mistake: When Morph is getting ready to attack Mr. Sinister, Sinister is standing in front of Magneto and Xavier. After his henchmen tackle Morph, Sinister then approaches Morph a different part of the room. (00:17:22)
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)
Continuity mistake: When Morph is laughing at Wolverine, the colors on the X emblem on his waistband have been inverted. (00:12:51)
Continuity mistake: When Morph, Jean, and Cyclops are all knocked unconscious by Ruckus' scream, Morph falls to his right, and Jean and Cyclops fall forward. A couple of shots later when their bodies are shown on the ground, Morph is suddenly between Jean and Cyclops, despite falling the opposite direction of them. (00:11:30)
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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