Continuity mistake: When we first see the padded envelope, Josh has put three stamps on it, spaced out. Then when he finally retrieves it, there are 8 stamps. Admittedly his friend might have replaced them with the correct postage, unlikely though it is, but unforgivably, the address is completely different - most noticeable is the middle line. The first time it's "5893 Tyler", and the second time it's "695 University Dr." The sender's address also changes from "Columbus St." to "549 Campus Drive".
Factual error: In the scene where the guys are in the cafe, the waiter pours coffee, then puts his hand underneath the hot coffee pot. You cannot do that with a true glass coffee pot if your coffee is hot. (00:43:30)
Continuity mistake: In the scene where the "fat" girl is sitting on the swing flirting with the geeky guy, she smiles and has perfect, straight teeth, in no need of braces. Later, in the scene where the geeky guy is introducing the fat girl to his dad at the door, the fat girl smiles and has braces on.