Other mistake: When Chris and Jamie are sitting on Jamie's bed, Tim comes in and shoves Chris onto the floor. This is supposed to be when the yearbooks are switched, but if you watch closely you will notice that Tim leaves with the same yearbook he brought in.
Continuity mistake: In the scene when Chris and Jamie are watching old home videos Chris has on braces, but when he brings her home he doesn't have them.
Deliberate mistake: In the beginning Chris and Jaime are on the bed talking and she is excited about graduating she is wearing a mini dress. In the shot where she falls face down in his lap and her dress goes up, she's wearing black tights underneath her dress, which are not there in any other shot.
Revealing mistake: After the first "day date", Chris gets into the Porsche and spazzes out while Jamie watches. She can't hear him because the windows are up, but as the camera moves into frame and into the car you can tell that the passenger-side window is down, presumably to allow for sound and camera entry.