Deliberate mistake: The Studebaker in which Carol is first riding bears the license number JPM 351, which interestingly enough, is not only the same license number that Steve's 58 Chevy has, but is also worn by the car Curt is sitting on when he first meets the Pharaohs. To top it off, the Edsel driven by Cindy Henderson has plates reading PRC 596, which are also used by a silver 57 Chevy and a red pickup.
American Graffiti (1973)
1 deliberate mistake - chronological order
Directed by: George Lucas
Starring: Harrison Ford, Ron Howard, Richard Dreyfuss, Charles Martin Smith, Paul Le Mat
Factual error: In one scene, Milner does a U-turn and you see a postal box on the curb. The logo on the box is the current white eagle drawing, but that was not used until well after 1962.
John Milner: So, your Judy's little... Shit! How old are you?
Carol: I'm old enough. How old are you?
John Milner: I'm too old for you.
Carol: You can't be that old.
Trivia: When Terry is being introduced, he drives up on his scooter, and has quite a bit of trouble parking it. This is because Charles Martin Smith actually didn't know how to drive a stick shift. The small accident he has is real, you can see him crash, look up towards the camera (at Lucas), and continue the scene. Lucas motioned for him to continue, because the crash looked so interesting.