Days of Future Past: Part 2 - S1-E12
Revealing mistake: When Gambit draws a card from his sleeve to use as a trap, the gloved part of his hand that is below the pocket is translucent. (00:09:28)
Family Ties - S4-E17
Revealing mistake: When Quicksilver is fidgeting inside his containment shell, the animators neglected to properly sync his body movement. You can see that his shoulders are not "connected" to his arms, as if his head and shoulders were separated from the rest of his body. (00:16:00)
Secrets, Not Long Buried - S4-E15
Revealing mistake: When the Lady Gardener shuts her door on Cyclops, his right arm stays inside her house in front of the door frame. (00:14:35)
Secrets, Not Long Buried - S4-E15
Revealing mistake: Instead of staying in place, the energy surging around Watchdog's head moves down the screen as the camera pans down. (00:02:35)
Revealing mistake: When Rogue's phone is ringing you can see that the phone is halfway absorbed into her mirror. (00:04:15)
Beyond Good and Evil: Part 1 - S4-E8
Revealing mistake: When Mr. Sinister is mocking Jubilee, take a close look at his neckline as he moves his head. The animators moved his neckline as he moved his head, but his shoulders stay in place, so his head and neck look like they are floating above his shoulders. (00:15:30)
The Dark Phoenix (4): The Fate of the Phoenix - S3-E14
Revealing mistake: When Xavier is invading Lilandra's mind, the ball of psychic energy on her head shifts left and right as the camera zooms in, likely indicating a composite shot. (00:02:07)
The Dark Phoenix (4): The Fate of the Phoenix - S3-E14
Revealing mistake: When Oracle attacks Jean, a beam of psychic energy emanates from Oracle's body. The beam then remains in place as Oracle moves towards Jean, likely indicating a composite shot. The beam is then back in place when the shot changes. (00:09:55)
The Dark Phoenix (3): The Dark Phoenix - S3-E13
Revealing mistake: After Wolverine tackles Jean, the glowing effect of the mnemonic scrambler device on her head briefly moves to a different part of her head. (00:15:08)
Dark Phoenix (1): Dazzled - S3-E11
Revealing mistake: When Dazzler steps off the stage and walks towards Cyclops, she briefly disappears from the screen. (00:16:20)
Dark Phoenix (1): Dazzled - S3-E11
Revealing mistake: When the Dark Phoenix is saying Cyclops might have to protect Dazzler from her, the glowing yellow effect of Jean's eyes are incorrectly placed around her eyebrows. (00:11:55)
Savage Land, Strange Heart (2) - S3-E9
Revealing mistake: When Storm is saying "Rain, lightning, thunder," the flashing green effect on her eyes sometimes appears on her cheeks instead. (00:02:21)
Savage Land, Strange Heart (1) - S3-E8
Revealing mistake: After Wolverine says "I'll take a smell-see," Jubilee jump-cuts towards him before she begins walking. (00:13:52)
Savage Land, Strange Heart (1) - S3-E8
Revealing mistake: When Storm is saying "I am free," the flashing green effect on her eyes moves down to her neck when she tilts her head back. (00:12:22)
Savage Land, Strange Heart (1) - S3-E8
Revealing mistake: When Zaladane is helping Sauron escape, she uses her staff to reveal a hidden stairway in the floor. As Sauron descends the steps, the opening fades back into resembling the floor to suggest it is being hidden again, but Sauron fades away as well. It should be that Sauron walks out of view, and then the opening fades back into the floor. (00:02:56)
The Phoenix Saga (2): The Dark Shroud - S3-E4
Revealing mistake: When Lilandra is saying to Xavier "I have seen your face a thousand times in my mind," the edge of an animation cel is visible around the parietal area of Xavier's head. (00:20:24)
The Phoenix Saga (3): Cry of the Banshee - S3-E5
Revealing mistake: When Wolverine is trying to break out of the crystal entrapment, a yellow box appears on screen for a few frames that says "Cut out these frames." (00:15:58)
Revealing mistake: When Beast is entering Carly's room to see if her procedure was a success, he passes through the door frame. (00:11:08)
The Phoenix Saga (2): The Dark Shroud - S3-E4
Revealing mistake: When Jean is saying that she feels strange, the edge of an animation cel is visible moving left and right under her pillow. (00:05:27)
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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