Continuity mistake: When Jean is using her telekinetic powers to rescue Senator Kelly from Master Mold's lab, she drops him on the floor in the middle of the lab. When Cyclops gets blasted by Master Mold after he and Jean run to grab Kelly, Jean and Kelly are suddenly inside the mine shaft. A couple of shots later, Jean and Kelly are now standing at Master Mold's feet. (00:15:42)
Continuity mistake: When Trask turns the laser onto the propane gas tank, it is on Master Mold's left. When Master Mold reaches down to try and stop the laser before it causes the tank to explode, it is now on his right. (00:16:04)
Continuity mistake: When Cyclops is running through the mine to search for Wolverine and Gambit, he uses his visor to track their location. When the shot changes to a first person view through Cyclops' visor, the background is merely moving up and down. If he was running as the previous shot had shown, the objects in the background from his point of view would become larger as he got closer to them. (00:16:50)
Continuity mistake: When Rogue is calling out to Gambit just before Jean and Senator Kelly exit the mine shaft, the X logo on her chest is a yellow X on a red background, when it should be a black X on a red background. (00:17:57)
Continuity mistake: At the beginning of Senator Kelly's campaign speech, his campaign button is predominantly white with some blue at the top and bottom. When the shot changes, the majority of the button is now blue, with only a small portion in the center being white. (00:19:20)
Continuity mistake: When Beast is first shown in his prison cell, the X logo on his waistband is black with a red background. When Jubilee hugs him, it is now a red x on a yellow background. (00:19:51)
Continuity mistake: Magneto reveals to Senator Kelly that he went to Washington, DC to personally witness his assassination. The flashback to the previous episode reveals that Magneto was watching Mystique (masquerading as Gambit) attempt to assassinate Kelly and the real Gambit's intervention through a window. Magneto can be seen as plain as day watching in the window in the flashback, yet in the actual episode, Magneto is never there. The design of the window is also different, as well as the color of the curtains. The room itself also appears to be smaller in the flashback.
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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