Continuity mistake: The position of Cyclops' body changes when Jean Grey is cradling his head. At first, he is sitting slightly upright. When the shot changes, he is lying down. (00:04:40)
Continuity mistake: The amount of damage to Cyclops' uniform decreases. When his Danger Room session is first terminated, his uniform is mostly missing around his shoulders and chest. A few shots later, the aforementioned area of his uniform is not as torn up. (00:04:15 - 00:04:50)
Other mistake: When the hotel clerk is checking Gambit, Storm and Jubilee into their room, he determines that they are mutants by seeing footage of Storm using her powers at the Salem Center Mall from the very first episode. While it's not unreasonable that security footage from the mall could exist showing Storm use her powers, the footage also shows an extremely close zoom-in on Storm's left eye, which is extremely unlikely, unless the security camera somehow guessed she was about to activate her mutant powers. (00:13:27)
Other mistake: When Wolverine, Rogue and Jean Grey are falling down the hole, they are all falling at the same rate, yet Wolverine lands a couple seconds after the other two. If anything, he should have landed first because he weighs more than them, especially since his bones are coated with metal, and Rogue and Jean Grey are shown to be slowing their falls by using their flight and telekinetic powers respectively. Wolverine also appears to land a couple feet in front of the hole.
Continuity mistake: The size of the hole beneath the trap door in the Thieves Guild's mansion changes. When Wolverine, Rogue and Jean Grey are shown falling down it, it is easily large enough for the three of them to fall down without bumping into each other. When we see the hole in the ceiling after Rogue destroys the wall that was falling on her, the hole is barely large enough for one person to fit through. The hole also moves closer to the back corner of the room, whereas it appeared to be closer to the back-center of the room once they landed.
Continuity mistake: The orientation of Gambit's mini-jet respective to the Blackbird changes. When the Blackbird first lands, the mini-jet is perpendicular to it. When Wolverine, Rogue and Jean Grey go over to inspect the mini-jet, it is suddenly parallel to the Blackbird.
Continuity mistake: Shortly after Wolverine, Rogue and Jean Grey enter the Thieves Guild's mansion, they fall down a trap door. However, the trap door is not there when they first enter the mansion, instead only appearing in the shot where they step on it.
Time Fugitives: Part 1 - S2-E7
Other mistake: When the Friends of Humanity member is going through pictures of the X-Men on his watch, Wolverine is shown to have blond hair.
Continuity mistake: When the X-Men regain their mutant powers, Jean Grey is suddenly wearing her uniform instead of the casual outfit she was wearing when she was abducted. In the next shot and for the remainder of the episode, she is back to wearing her casual outfit.
Days of Future Past: Part 2 - S1-E12
Other mistake: Wolverine is around 5'3", but when Forge is viewing his skeleton in the tube at the end of the episode, the skeleton appears to be at least seven feet tall.
Other mistake: When Bella Donna is saying that Gambit was wise to leave the Thieves Guild, there is a black horizontal line on Gambit's right cheek that disappears and reappears in sync with his blinking.
Continuity mistake: Snow suddenly appears on Wolverine's shoulders when he reaches the bottom of the ski slope.
Continuity mistake: When the camera pans around the team after Lilandra saves Xavier, Storm's communicators are colored white whereas they are normally red, and Rogue's gloves are missing below the wrist. (00:18:50)
Continuity mistake: When Jean Grey is stating how Magneto's powers intensified her own, the left side of her costume around her abdomen is colored yellow whereas it is usually blue (the right side remains blue in this shot). (00:13:30)
Continuity mistake: When Magneto is saying "You are the next step in man's evolution," the lower half of Sunfire's mask is flesh-colored. (00:11:55)
Audio problem: When Magneto is saying "I have heard your cries for justice and I now answer that call," his mouth is not moving. (00:11:45)
Continuity mistake: When Trish Tilby is giving the news report, she is first shown to be in front of a gray news van. When the shot changes and for the rest of the report, she is standing in front of a blue screen. (00:05:30 - 00:06:45)
Continuity mistake: When Cyclops, Wolverine and Gambit are watching Morph giving a statement on the news while masquerading as Xavier, Cyclops' hair looks like a bowl-cut rather than the usual semi-shaggy look he sports. (00:06:20)
Continuity mistake: When Moira is asking the X-Men how they intend on contacting Lilandra, Jean Grey's face is colored yellow. (00:05:10)
Answer: It appears a number of factors led to its demise. It was originally intended to air for 65 episodes, but its popularity extended that. However, there were continual production quality problems, issues regarding whether the content was suitable enough for children, as well as financial considerations that finally led to it being ended.
raywest ★