Continuity mistake: After Magneto crashes through the roof of the U.N. building, the nameplates designating the nations of the delegates have suddenly become blank. (00:02:20)
Audio problem: Magneto's mouth is closed when he says "The mutant race will no longer be managed, mister delegate." (00:02:20)
Continuity mistake: When Magneto forces the Genoshan delegate to sit down, the chairs surrounding him are empty. A couple of shots later, the chairs are occupied. (00:02:35)
Plot hole: Some of the footage Magneto is showing the delegates at the U.N. meeting includes footage Magneto could not possibly have had access to even if such footage existed, such as the X-Men having mission briefings in the War Room, the Morlocks living in the sewers or Warren Worthington emerging from being buried under several feet of snow at his secluded home in the mountains. (00:03:10 - 00:04:05)
Continuity mistake: When Magneto is showing Asteroid M to the delegates at the U.N. meeting, the Genoshan delegate he forced to sit down has either moved to another seat entirely or the people that were sitting around him moved to different seats and other people filled in their seats. (00:04:05)
Audio problem: Fabian Cortez's mouth is closed when he says "Lord Magneto." (00:10:15)
Continuity mistake: When Gambit is trying to distract the Sentinels, both of his gloves are entirely fingerless whereas normally his gloves are only fingerless for the thumb, index and pinky fingers. (00:12:00)
Audio problem: When Xavier is saying "I fear Magneto may come to depend upon it like a drug," Beast is actually the one mouthing the line. (00:14:15)
Continuity mistake: A crowd of people has suddenly gathered in front of Magneto when Xavier says he wishes to speak with him. (00:14:20)
Continuity mistake: When Magneto first notices the missiles being fired at Asteroid M, he is dressed in loungewear and Xavier is wearing his usual green suit. When the shot changes Magneto is suddenly wearing his costume and Xavier is wearing a more casual outfit. Given how fast the missiles are shown to be approaching, they wouldn't have had time to change their clothes. (00:15:45)
Continuity mistake: The missiles that are fired at Asteroid M are first shown to be approaching from Earth. When the shot changes, they are coming from the opposite direction. When the shot changes again, they are once again approaching from Earth. (00:15:45)
Factual error: The Moon is more than 230,000 miles away from Earth, yet in a shot of Asteroid M just outside of Earth's orbit, the Moon is right there as well. (00:15:50)
Audio problem: Beast's mouth is wide open when he says "Feet don't fail me now." (00:02:05)
Factual error: When Rogue attempts to correct the space shuttle's flight path, she has to let go because of the intense heat due to re-entry. Indeed, a temperature as hot as a shuttle in re-entry (upwards of 3,000° F) would have completely melted through her gloves, yet when she lets go, they only have a little smoke coming off them. (00:03:40)
Factual error: Even though Xavier is a paraplegic, you can see him freely moving his legs when the astronaut helps him out of his seat and walk over to Beast. (00:04:20)
Continuity mistake: When Rogue is asking about Gambit's whereabouts, she along with Beast and Xavier are close to the front of the shuttle. When the shot changes, the three of them as well as one of the astronauts are suddenly further back in the shuttle (note the doorway). (00:04:25)
Continuity mistake: When Fabian Cortez is scapegoating Gambit in front of the mob, he grabs him by the blue clothing that lines the body armor on his chest. When he says to Gambit "Mark my words X-Man..." the blue clothing is now the same brown color as Gambit's trenchcoat. (00:05:20)
Audio problem: When Fabian Cortez says "Mark my words X-Man. Before this day is done you will beg me to let you join us," his mouth movement is out of sync with the words "is done." (00:05:40)
Continuity mistake: In the flashback during Magneto's memorial, when the Nazis are approaching his home it says "Lehnsherr" on the door. After they kick the door open, the writing on the door has disappeared and either a hatch or a small window has appeared on the door from nowhere. (00:06:20)
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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