In the opening of the movie, when the guys are young, there is a Camelback on the wall in the brother's room. The Army pattern on the Camelback is the ACU pattern that wasn't in use during that time period. The Army adopted that pattern universally in 2005.
There is a scene where Tucker has to dodge a car that he almost hits while driving, however, before and after the dodge, the headlights of a car driving behind them are seen in the background, but when he dodges the car in front of him, the headlights on the background don't move.
When Tucker lights up the pipe, we see a little burning piece of ash float to the curtains in the back, and light them up. However, there are three mistakes. First, the little burning ash turns off right before touching the curtains, second, the fire comes from below, instead of coming from where the ash supposedly touches the curtain, and third, a curtain could not be lit up in that way, and that fast by a little burning ash, unless it had some fuel poured to it previously.
The goof items below may give away important plot points.
When Eugene picks up Cindi at her parents place for the prom, one of the girls pops out of the limo and spits out semen she supposedly got from Horsedick.mpeg. At the end of the film, we learn that Horsedick.mpeg doesn't have genitals.