Continuity mistake: Right after Lloyd gets his nose broken while kickboxing, Diane begs him to take her back. As they converse, the string of drying blood under Lloyd's nose changes from being to the right of his nostril to directly under it.
Factual error: When Diane visits her father in prison, she hands him a pen, but visitors in prison are very closely searched - no way could a visitor give a prisoner a pen of any kind (they're contraband for prisoners as they can be used as weapons), let alone a heavy pen like that one. Even if he were in a minimum-security prison (which his orange jumpsuit seems to indicate he's not) this rule would still apply.
Trivia: The Smithereens were commissioned by Cameron Crowe to write the theme song for the movie, and they came up with "A Girl Like You." Crowe thought that the lyrics were too leading (they outline the entire plot), so he rejected it in favor of Peter Gabriel's "In Your Eyes." "A Girl Like You" went on to be included in the Smithereen's next album, "11."
Lloyd Dobler: Why can't you be in a good mood? How hard is it to decide to be in a good mood and be in a good mood once in a while?"
Constance: Gee, it's easy.
Mike Cameron: I don't know you very well, you know, but I wanted to ask you - how'd you get Diane Court to go out with you?
Lloyd Dobler: I called her up.
Mike Cameron: But how come it worked? I mean, like, what are you?
Lloyd Dobler: I'm Lloyd Dobler.
Mike Cameron: This is great. This gives me hope. Thanks.
Lloyd Dobler: What I really want to do with my life - what I want to do for a living - is I want to be with your daughter. I'm good at it.