At the scene where the two detectives are about to drive Sidney and her friend away from the college, Sid stops to talk to Derek and her friend gets in the car and slides across to to left (looking from the front), Sidney gets in and sits on the right. The scene then goes to Derek at a party, bla bla bla. When we see Sidney and her friend next, they've swapped places in the back of the car! One would also think that if Sidney's friend (i can't remember her name) couldn't break the back window, the killer couldn't simply smash the front window by simply punching it.