Continuity mistake: When Jean & Michel are playing their guitars on stage with their band. They see Melanie & Ally at the dance, they put down their guitars and go over to them. As the twins are talking to someone else, you can see them in the background pointing to them & getting off the stage again.
Continuity mistake: In the end at the dinner party, they are walking down the stairs and Mary-Kate (Melanie) has one piece of hair sticking out really badly, but in the next shot, her hair is perfectly fine.
Continuity mistake: When they arrive at the mansion Ashley (Ally) starts to take out a brush from her backpack, then in the next scene her bag is closed, then in the scene after that she's taking out the brush again.