Revealing mistake: When Bert is running in the woods, away from the local guys, he rubs blood on a tree to make them believe he ran a certain way. Look close, the tree already has red tint on it probably from previous takes that didn't wash off. (01:04:35)
Revealing mistake: Despite her very vigorous thrusting when she makes love with Paul, the bed sheet covering Marcy's ass doesn't get shaken off. Not even a little bit. Also, when they finish, she collapses on to him and he wraps his arm around her back. When he does this you can see that his arm is coming up from near the bottom of her waist. If his arm lines up with her lower waist, then Marcy's crotch must line up with his chest or belly - much higher then it needed to be to have sex. (01:02:40)
Revealing mistake: When Jeff walks out the cabin and is shot by police, you can see the squibs bulging underneath his clothing. (01:22:00)
Revealing mistake: When Marcy cleans Karen's face up after Karen had vomited blood, the paper towels she uses don't absorb any of the blood, and stay pure white. (01:00:50)
Revealing mistake: Just before Paul walks into the bathroom to disinfect himself, the bathroom light turns on all by itself. (01:02:50)
Revealing mistake: When Marcy and Jeff are having sex, at one point Marcy rolls over on top of Jeff. When she does this, there's a couple of instances where you can see a red cord from the bikini bottom she's wearing to preserve her modesty dangling down in the background. (00:14:05)
Revealing mistake: As Marcy is running away from the mad dog, there are a few instances when we can see the sleeve of a white top inside her bathrobe. She was supposed to have just run out of the shower in a panic, so throwing on a top and then a bathrobe doesn't seem like something she really would've done. (01:10:55)
Revealing mistake: There's no chimney on the roof, but the fireplace works.
Revealing mistake: In the cave scene, when Paul finds Grim's body, during the "worm in the ear" shot, Grim's neck is moving quite visibly with his pulse. (01:11:20)
Revealing mistake: When the police are throwing the bodies on the fire near the end of the film, the body that gets thrown on top is obviously a doll. The body on the bottom looks like it possibly could be, but the one on top is definitely one. (01:25:50)
Answer: Sticking her finger up his bottom. I believe it is mentioned in one of the commentaries.