Revealing mistake: When Xavier is parachuting towards the ground, the stars in the sky are flickering on and off. (00:19:00)
Factual error: When a parachuting Xavier is about to land, you can see his legs are bent at the knee to brace for landing. Since Xavier is a paraplegic, he shouldn't be able to do this. (00:19:14)
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)
Factual error: When Senator Kelly is giving his campaign speech, the U.S. flag behind him is missing one stripe and has at least two too many stars. (00:19:30)
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)
Plot hole: Magneto states that he went to Washington, DC to witness Senator Kelly's assassination. But Magneto had no way of knowing about the assassination plot. Mystique was the one who was attempting to assassinate Kelly, and Apocalypse was the one who carried out the order. At this point in the show, Magneto had not worked with either Mystique or Apocalypse, and it's highly doubtful that Mystique would leak the plot, since it was supposed to be covert.
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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