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.)
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.