Revealing mistake: When the boys drive the car over the gap in the bridge, look at the side view that shows the main jump, watch the front wheels, they lift up in the air unnaturally before the car leaves the ramp. (00:34:00)
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".
Continuity mistake: When the band is playing in the background at El's house you can clearly see that one of the band members has a cigar in their mouth, but when the camera cuts to the band playing the cigar is gone.
Continuity mistake: Twice towards the end of the film (as we see the bus arriving at Austin, then also as it arrives back home) the bus has a front number plate (clearly visible on the front bumper during the montage driving shots), then soon afterwards it disappears.
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.