Continuity mistake: When Gambit is saying "Someone may be tracking us," the snaps on both shoulders of his jacket are colored blue, when they should be the same brown color as the rest of his jacket. (00:11:45)
Character mistake: When Rogue asks Storm what her plan is for dealing with the closing walls, Storm says "You three stop the walls, I shall slow the ceiling." Besides Storm and Rogue, Jubilee, Jean Grey, and Gambit are also there, so Storm should have said "You four stop the walls." (00:03:12)
Continuity mistake: When Gambit is saying "Scott and Jean pick one strange romantic hideaway," the encircled X on Rogue's right shoulder is entirely red, when the X and the circle around it should be black. (00:11:39)
Continuity mistake: When Callisto is saying "Now, for the animal," part of Cyclops' brown pants briefly turn white. (00:17:13)
Continuity mistake: When Wolverine is slashing at Callisto's staff, it is light purple. When he completely shreds a center portion of it, that portion is blue, and when the shot changes, the rest of the staff is red. Additionally, the center section of the staff has reduced in length after Wolverine shreds it. (00:14:35 - 00:15:03)
Continuity mistake: When Wolverine is saying "Cyke has his problems, but he don't smell like a Morlock," Gambit's bare neck is visible, when it should be colored black because of his headgear. (00:14:11)
Continuity mistake: When Jean is saying "Where am I? What's happening?", she has a lock of hair draped over her left shoulder that suddenly turns a darker shade of red. (00:13:26)
Continuity mistake: When Wolverine punches his lefthand claws into the wall, it is at bottom of a slab of concrete, and to the left of where four slabs intersect. When the shot changes, the puncture is now closer to the middle-right area of a slab. (00:13:16)
Continuity mistake: When the X-Men discover Jean and Annalee in the small room, neither Jean or Annalee are casting shadows on the wall behind them. When the shot changes, their shadows are now on the wall. (00:12:37)
Continuity mistake: When Leech telekinetically splits up the mob, on his left are a man with a light blue shirt, followed by a man with an orange shirt, and man with a light blue shirt behind him. On his right are a man with a green jacket and yellow shirt, a man with an orange jacket and pink shirt behind him, and lastly, a man with a blue jacket and yellow shirt. When the shot changes, the men have not only swapped sides, but now it is two men in light blue shirts in front of the man in the orange shirt, and the man with the orange jacket and pink shirt is now just wearing an orange shirt, and he is standing in front of the man with the green jacket and yellow shirt, and the man with the blue jacket and yellow shirt is in front of him. (00:06:05)
Continuity mistake: When Leech begins to telekinetically steal fruit from the vendor cart, he is stealing a red apple and what appears to be an orange with a yellow peel. When the shot changes, the orange is now orange in color, and as the two pieces of fruit move towards Leech, the orange is on his right, and the apple on his left. When the shot changes, the orange is now yellow in color again, and the fruits have swapped sides. (00:05:52)
Continuity mistake: After Jean and Cyclops first kiss after she tells him to relax a little, his left arm is around her shoulder and it suddenly jump cuts to his side with his hand in his pocket. (00:05:37)
Continuity mistake: When Wolverine finds the unconscious Cyclops, Cyclops' sleeve is rolled all the way down. A couple of shots later, the sleeve is now rolled halfway up. (00:16:42)
Deliberate mistake: The video footage Callisto has of the X-Men battling Sentinels at the Mutant Registration Center is just clips from that episode. (00:09:27)
Continuity mistake: When Tar Baby traps the mob inside the subway platform, he seals off the stairs with a wall of tar. When Cyclops and Jean have arrived to intervene, the wall of tar is now in front of the turnstiles. (00:06:24)
Continuity mistake: When Xavier is talking to Jean and Cyclops at the end, the three of them, along with Storm, are all to the left of the bed Gambit is lying on, with Storm being behind Xavier, over his right shoulder, and a piece of machinery behind her. When the shot changes, it is a closeup of Xavier, and neither Storm, nor the machinery are visible over Xavier's shoulder. When Jean and Cyclops leave the room, Xavier is now at the foot of Gambit's bed, several feet away from Storm. (00:20:08 - 00:20:32)
Continuity mistake: The picture of Jean and Cyclops Wolverine has framed shows them against a yellow-orange background, and Cyclops' sunglasses are red. When the picture is shown again at the end of the episode, the background is now white and Cyclops' sunglasses are gray. (00:08:14 - 00:20:44)
Continuity mistake: When Storm is offering the Morlocks sanctuary at the mansion, several of the Morlocks that were standing dozens of feet behind Storm in a previous shot are now standing in front of her. They are then behind Storm again when she is helping Callisto to her feet. (00:18:58 - 00:19:45)
Continuity mistake: When Storm defeats Callisto in combat, Sunder and Tar Baby are in the far background, standing near a tunnel opening. When the shot changes, they are now standing in front of some machinery. (00:18:58)
Continuity mistake: When Storm and Callisto are about to engage in combat, they are using staves that look like lightsabers, with the hilts at the bottom. When they start striking at each other, both staves now resemble a double-edged lightsaber, with the hilt in the middle. (00:18:25)
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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