One Man's Worth: Part 1 - S4-E1
Other mistake: Shard shows Storm and Wolverine surveillance footage of Trevor Fitzroy's deal with Master Mold, stating it was all they could get of their plan before the camera was discovered. However, the footage shows multiple camera angles that a lone mounted surveillance camera could not achieve. (00:11:30 - 00:12:35)
Other mistake: When the two civilians are walking past Joe McTaggert's campaign posters, they are shown to be walking unusually fast, almost as if the ground beneath them is moving them forward between strides (similar to as if you were walking on a moving sidewalk inside an airport). (00:03:00)
Other mistake: Moira's arms are disproportionately long when she rescues Xavier from falling into the pit. (00:12:30)
Other mistake: When Cortez is using the remote control for the power inhibitor collar to fend off Gambit and cause him pain, you can see that Cortez is actually pushing the "Mutant Power On" button and not the "Pain" button that's just above it. (00:16:25)
Beyond Good and Evil: Part 4 - S4-E11
Other mistake: During the montage of people being erased from time, the exterior shot of the X-Mansion shows part of the mansion destroyed. The mansion is later shown to be destroyed by Graymalkin after the heroes defeat Apocalypse, but at this point in the episode there's no reason for the damage. (00:12:20)
Have Yourself a Morlock Little Christmas - S4-E12
Other mistake: Rogue states that this is Jubilee's first Christmas as an X-Man. However, Jubilee was already a member of the X-Men when Senator Kelly declared his candidacy for President in season 1, and the season 2 premiere episode showed him being sworn-in as President. Election Day is in November and the President is sworn-in in January, so Jubilee should have already had at least one Christmas as a member of the X-Men. (00:02:00)
Have Yourself a Morlock Little Christmas - S4-E12
Other mistake: When Ape and Annalee walk to the back doors of the ambulance, the window on the right rolls down. Ignoring the fact that those windows don't actually roll down, neither Ape nor Annalee do anything to make it roll down. (00:06:55)
Have Yourself a Morlock Little Christmas - S4-E12
Other mistake: Jubilee should not have been able to see Mariana without looking around the corner. (00:10:25)
Have Yourself a Morlock Little Christmas - S4-E12
Other mistake: When Gambit is tasting the food in the crock pot, his left arm has been drawn backwards.
The Lotus and the Steel - S4-E13
Other mistake: When Wolverine is telling Xavier about his road rage incident, his jeep is missing its windshield. (00:03:20)
The Lotus and the Steel - S4-E13
Other mistake: The camera is out of focus when the Silver Samurai threatens the villagers. (00:09:35)
The Lotus and the Steel - S4-E13
Other mistake: The Silver Samurai slashes Wolverine in the back with his sword twice and Wolverine reacts in pain, yet there are no tears in Wolverine's shirt afterwards. (00:19:05 - 00:19:50)
Other mistake: The tail that Rogue grows appears out of thin air as if it were magic. Given that she's going through an organic transformation, the tail should have formed gradually out of her coccyx. (00:15:55)
Other mistake: After Rogue absorbs Wolverine's healing powers and purges the transformation spores from her system, she reverts back into human form including losing the tail that she grew. Given that the tail protruded from her backside, there should have been a hole in the back of her pants afterwards, yet there is none. (00:16:40)
Secrets, Not Long Buried - S4-E15
Other mistake: At one point after Cyclops ejects from his mini-jet, you can see that he is just floating in midair instead of descending back to the ground. (00:03:15)
Secrets, Not Long Buried - S4-E15
Other mistake: When the group of mutants are listening to Solarr's speech, in one continuous shot of ten of them that lasts at least four seconds, they are all standing completely still, not one of them moving a muscle, blinking or making the slightest adjustment in their facial expressions. (00:17:00)
Other mistake: When the Shadow King in his human form is passed out on the floor, his left hand appears to have at least six knuckles. (00:08:25)
Family Ties - S4-E17
Other mistake: When Scarlet Witch falls from the cliff, she lands on a hover craft and kicks its pilot off. The pilot is then immediately show falling into a pond. It then takes Scarlet Witch several seconds to save Quicksilver who was free-falling from the same cliff. Given how fast the hover craft pilot fell to the ground, Quicksilver also would have fallen to the ground before Scarlet Witch could have caught him. (00:16:50)
Family Ties - S4-E17
Other mistake: All of Scarlet Witch's screams in this episode are recycled from Lady Deathstrike screaming in the season 3 episode "Out of the Past, Part 1." (00:16:55)
Family Ties - S4-E17
Other mistake: The final shot of Magneto flying away is a recycled shot from the season 2 episode "Reunion, Part 2." (00:20:40)
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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