Revealing mistake: When the killers first pull out Phyllis's severed arm to show Mari, the fingers are moving.
Visible crew/equipment: When the girls go in Krug's hideout and he starts pushing them around, Junior steps in to help them and he gets knocked backwards. When this happens you can see the cameraman push him in the back so that he wont fall into the camera.
Revealing mistake: Mari breathes and moves when her parents find her body (the original intention was that she should still be alive at this point and would identify her attackers before dying).
Continuity mistake: When Weasel and Sadie are chasing Phyllis through the woods, Sadie takes off her jacket and throws it on the ground. It later reappears and then disappears again.