When the drunk fat guy falls down in Dawson's flashback, his hat falls off. In the next shot, the hat is back on his head.
As lunch continues, the pattern on Martina's sake flask changes between shots.
After Dawson hits Hag and she's sitting in the van, she wipes of some of the blood on her chin. In the next shot it is all there again untouched.
When Barbara is speaking to the school nurse, you can see the door behind the nurse has a pocket on it with a black magazine/book sitting in front of a Manila folder/envelope. Towards the end of their conversation, before standing up, the black book/magazine disappears, and there is only the Manila folder/envelope.
Martina is asked to bring her grandma laxatives. She only grabs her keys and leaves the box of laxatives on the drawer, but when she and her friends dump the deer a little while later she pulls the box out of her jacket pocket.
At one point in the movie, Boner says, "Roofies are bull. I take them before every date, and I never get laid." If Boner actually did take roofies before every date, he could have gotten laid on every single date and not remembered since roofies make you forget what happened whenever you took them.
You can see that the killer is actually wearing a flesh-colored thong in the shower, when he's supposed to be naked.
When the Killer pops out of the "cake" at the beginning of the movie, the right side of the cake wobbles like cardboard.