Other mistake: When Mary-Kate and Ashley arrive at the dance, the two boys are up on stage with their band. They are both playing the guitar, but when they see the twins they put down the guitars and go to see the girls. The thing is that the music sounds exactly the same before they leave as after they leave.
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.