Continuity mistake: When the Thieves and Assassins guilds begin to skirmish, they start by trying to regain the weapons that Jean Grey telekinetically confiscated from them. A couple seconds into the fight and most of the weapons have disappeared altogether.
Continuity mistake: When Gambit is looking at himself in the mirror while wearing a tuxedo, you can see in his reflection that he is wearing pinkish-purple gloves (or is barehanded, it's not entirely clear). In the next shot when he removes the tuxedo, he is wearing his usual black gloves. Also, the rose on the tuxedo goes from being on his right side (reflection's left) to being on his left side just before he removes the tuxedo.
Continuity mistake: In the first establishing shot of the Assassins Guild's mansion, there is a stairway leading up to the front door. In the establishing shot of the mansion in the scene where Gambit is in Bella Donna's bedroom, the stairway is missing, and consequently the mansion's porch area isn't as high off the ground.
00:09:20 - 00:13:20Phaneron
Continuity mistake: When the chosen one for the Thieves Guild tries to present their tithe to the External, he is shown walking right up to the tree stump where he was to have placed the tithe. When the shot changes, he is still walking forward, and therefore should've bumped into the stump. He is saying a line of dialogue as he is walking forward, so it can't be considered an action shown from a different angle.
Continuity mistake: When the chosen one for the Thieves Guild is called forward to present their tithe, the chosen one for the Assassins Guild can still be seen kneeling before the External. In the next shot when the chosen one for the Thieves is shown arriving on the scene, the chosen one for the Assassins is suddenly back to standing among the other members of his guild. In the next shot, the chosen one for the Assassins is back to kneeling before the External.
Continuity mistake: During Gambit's flashback, the chosen one for the Assassin's Guild is shown to be placing their tithe box on the stump of a tree roughly halfway between the center ring and the outer edge. When the shot changes, the tithe box has suddenly moved to the center of the stump, and the tithe box itself is suddenly wider and shorter and has rotated 90°.
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 - 00:04:50Phaneron
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.