Continuity mistake: When Bert and Jeff find the crazy woman who is gutting the pig, when she first starts yelling at them, there is blood all down the side of her protective goggles, but then suddenly it all disappears and her goggles are completely clean. (00:31:20)
Continuity mistake: When Bert is out shooting squirrels, he has a bottle of beer in a brown glass. But when he starts the fireplace up, the bottle is green. Also in certain shots as he drinks from it, it's clear the bottle is empty. (00:14:50)
Continuity mistake: When Paul begins telling the story of the mass murder in his town's bowling alley he says the alley was called "Lenny Mead's Brighton Bowl". But when we see the flashbacks of the massacre, the business name embroidered on the employees' shirts is "Black Sky Bowling" (00:19:00 - 00:21:05)
Continuity mistake: The cabin interior doesn't fit with its exterior. In the interior shots you can see that the front door opens in to the main living room, with a window beside it. Next to this window is a doorway to the main bedroom. In other shots, we see that this bedroom also has a window on that same wall. In the exterior shots, there is only one window on this wall, and the cabin's length is nowhere near long enough to fit two spacious rooms. (00:54:40)
Continuity mistake: When Karen and Paul are on the raft, they are laying down talking. In a specific overhead shot Karen's arm are by her sides, but in the next shot they are folded over her body. (00:13:50)
Answer: Sticking her finger up his bottom. I believe it is mentioned in one of the commentaries.