Continuity mistake: Callisto's staff is roughly the same length as her body. When she is holding the staff to Cyclops' throat, only the bottom quarter of it is visible, when the rest that isn't obscured by their bodies should be visible and pointing outwards at her left. (00:18:06)
Continuity mistake: When Wolverine drops Cyclops to the ground, Cyclops' body is at a roughly 45° angle to the tunnel entrance. When Jean intervenes, Cyclops' body is now parallel to the tunnel entrance. (00:17:19)
Continuity mistake: When Wolverine returns to Storm and Jean, you can see no-one is in his immediate vicinity. A couple of shots later, Callisto and Annalee are standing by the tunnel entrance, a few feet to Wolverine's left. (00:17:05)
Continuity mistake: When Wolverine returns to Jean and Storm after finding Cyclops, he is standing in the opening of the tunnel. When the shot changes, he is standing near the ladder that was several feet behind him in the previous shot. (00:17:05)
Continuity mistake: When Rogue is flying Gambit back to the mansion, the green sides of her uniform above her belt are incorrectly colored yellow, and Gambit's neck is bare, when it should be colored black because of his headgear. (00:16:14)
Continuity mistake: When Rogue is checking on Gambit, the black X's on a red background on both of her jacket sleeves are colored entirely red. (00:16:02)
Continuity mistake: When Storm is blasting down the door to the small room Jean is captive in, there is a small vent cover to the left of the door. When the shot changes, the vent cover disappears, and then reappears in the next shot. (00:12:33)
Revealing mistake: When Gambit and Wolverine are pinned to the wall by Tar Baby, Gambit's face somehow gets between the tar and his right shoulder when he is saying "Sticky situation, mon ami." (00:12:01)
Other mistake: When Xavier is in the War Room with the rest of the team showing where the closest he could pinpoint Cyclops and Jean's location is, he is sitting in one of the chairs positioned around the table. Xavier is paraplegic and usually sits in his own hover chair. While there's no reason he couldn't be sitting in a regular chair, it would be very impractical for him to do so, especially if he's just instructing his team where they need to go. (00:11:07)
Continuity mistake: When the mob was first chasing Leech, it was six men. When they have reached the subway platform, it is now four completely different men and one woman. (00:06:05 - 00:06:28)
Continuity mistake: When Cyclops and Jean are approaching the newspaper kiosk, its opening is facing the sidewalk. When the shot changes to them walking past it, the opening has been replaced by a wall with newspaper clippings and a painting on it. Moments later, they are approaching the same kiosk, after he tells her that his mind is on nothing but her. (00:05:13)
Continuity mistake: When Jean and Cyclops are kissing after he tells her his mind is on nothing but her, he is wearing a gray suit jacket, when he was only wearing a yellow collared shirt in all previous shots. (00:05:05 - 00:05:48)
Continuity mistake: When Jean is first shown in her dress, it is a solid pink color. When she is walking down the sidewalk on her date with Cyclops, the midsection of her dress is now a lighter shade of pink than the rest. (00:04:34 - 00:05:18)
Continuity mistake: When Jean is walking out of Wolverine's room, part of her dress around her abdomen briefly changes to a lighter shade of pink. (00:04:57)
Continuity mistake: When Leech steps in front of Cyclops and Jean to steal fruit from the vendor, you can see the street behind him. When the shot changes, there is suddenly what appears to be a covered bus stop behind him. (00:05:48)
Continuity mistake: When Jean Grey is first shown standing in Wolverine's doorway, her arm is resting on the door with her elbow level with her shoulder. When the shot changes, her elbow is now level with her breasts. (00:04:34)
Continuity mistake: When Rogue is thrown into the wall by Storm's wind, the green side of her uniform is incorrectly colored yellow. (00:03:52)
Continuity mistake: When the walls begin closing in on the X-Men in the Danger Room, the blue center of Jean Grey's outfit is incorrectly colored the same light-orange color as most of her outfit. (00:03:12)
Revealing mistake: When Wolverine, Storm, Rogue and Gambit are walking down the stairs to the subway station, you can see that they aren't actually descending the steps, but walking on thin air. (00:11:10)
Audio problem: When Callisto says to Cyclops "away from those who fear us," her mouth is completely closed. (00:09:20)
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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