Continuity mistake: Towards the end of the movie during the play, Kelly goes backstage during intermission, and her hairstyle and color has changed. (According to DVD commentary, it was a pick-up scene and she had already started filming Spider-Man. That's why her hair was now red. Still a mistake, but there's the explanation.)
Continuity mistake: When Mr. and Mrs. Landers are demonstrating something for their show, she at first has a sparkly type shirt on, then afterward she has a green button down shirt on.
Continuity mistake: When Burke and Kelly kiss at the party, Felix punches Burke with his right hand hitting Burke on the left side of his face. The next scene as Burke is walking and meets his parents, he is holding the right side of his face, which is not where he was hit.
Audio problem: In the club scene toward the beginning of the movie, Kelly and Basin are talking to her brother Felix. Kelly says "Come on, there's a guy over there who says he'll pierce our navels for free." But if you look closely, you can see that Kelly actually says "pierce our nipples for free."
Continuity mistake: Toward the beginning of the movie, in the scene where his friends take Berke to the club to get over Allison, Felix spots Kelly and Basin dancing. Kelly is wearing a sparkly cardigan over her tank top, but in the very next shot when Felix approaches her, the sweater is suddenly gone.
Continuity mistake: All the way through the movie (apart from in the play) Kelly wears a blue bracelet on her left wrist. In the scene where her hands are shown close up playing the piano it is on her right wrist.