Continuity mistake: When Rat approaches Stacy at the check-out to ask for her phone number, the characters in the background keep changing. Sometimes you see Damone standing there; other times there are different people.
Plot hole: In the very beginning at the mall, where the older guy comes in to the pizza shop and sits in Stacy's section - the co-worker says "she's not even in high school yet." First of all, the school she's at is "Ridgemont High", and secondly, if she wasn't in high school (which would mean she would be younger than 15 at least) - how could she have a job, where the minimum age is at least 15? Even by mall standards, it's doubtful they would hire someone who is 14.ckbyers
Other mistake: In the scene when Mark Ratner asks for Stacy's phone number, there is a customer paying for a slice of pizza. Stacy tells the customer to pay $1.10. The customer gives her a dollar and change. She then gives him five cents back. The only way this could be right is if the customer gave her $1.15. If he could do that, why not the exact $1.10?