Continuity mistake: When Nightcrawler teleports into the police car, the seat belt is buckled around him. How did the seat belt buckle itself around him? The police wouldn't have buckled the seat belt for no reason.
Mainstream - S3-E3
Continuity mistake: When Kitty goes to get a midnight snack, her hair is down. But a few shots later, her hair is up in a ponytail.
Mainstream - S3-E3
Continuity mistake: During the cold open, Kitty is wearing slippers when she and Kurt fight the vandals. After the battle, she's barefoot, with no explanation as to where her shoes went.