Say Anything... (1989) - 4 mistakes

Continuity: When Lloyd calls Diane's house for the first time to ask her out, he gives her father his phone number as 555-1342, but later when Diane is reading her list of messages, Lloyd's number is listed as 555-2342.

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.

Continuity: 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.

Continuity: When Lloyd and Diane are driving during the scene where Diane breaks up with him, Lloyd is continuously looking at Diane for very long periods of time without looking back at the road. It is very obvious, and surely he would have hit something eventually.