Continuity mistake: When Paul squeezes Marcy's back during sex, his fingers are pointed more inwards towards her spine then upwards towards her shoulders. The hand-shaped rashes Marcy notices in the bathroom immediately afterwards reflect this position, also. But as the rashes deteriorate, the finger marks shift into positions that are completely different to the way Paul was holding her. (01:02:35 - 01:10:30)
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: Just before Paul walks into the bathroom to disinfect himself, the bathroom light turns on all by itself. (01:02:50)
Continuity mistake: After Paul uses the Listerine as a disinfectant, there's a closeup on the bottle and you can see that the level of the Listerine left inside is just below the word "Listerine" on the label. Every time the bottle is seen after that, the level of liquid is higher. (01:03:00 - 01:10:25)
Continuity mistake: The way Marcy is holding the towel when she notices the rashes on her back changes in the closeup. (01:03:10)
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)
Audio problem: In the scene where Marcy tells Paul that he hurt her back (when they had sex) you can hear a clock ticking in the background. But in the scene a few minutes earlier where she talked him into having sex with her, that takes place in the same bedroom, the sound isn't there. (01:05:45)
Character mistake: Paul tells Marcy he's leaving her to go look for Jeff. But soon afterwards, when Paul hears Marcy's dying screams, he runs back to the cabin from exact opposite direction to the one Jeff went when he ran off into the woods. Paul watched Jeff leave, so he would've known what direction he should've been searching in. (01:05:50 - 01:11:10)
Continuity mistake: When Marcy is shaving her legs, in a close-up shot of her face we see a small smear of shaving cream on her forehead that hadn't been there in the previous close-up. A few shots later, it has vanished just as mysteriously as it appeared. (01:09:40)
Continuity mistake: When Marcy jumps up in the bath/shower after shaving the flesh off her legs, we see a lot of blood pooled around the wounds on her back when only seconds ago we saw that the wounds were scabby, but not bleeding at all. Not only that, but somehow the blood has managed to spill upwards, reaching several inches above the wounds it came from. Also, when she begins to jump up, she's facing the shower head directly, but when the camera angle changes, she springs up facing towards the bathroom mirror, which is about a 45 degree angle difference in direction. (01:10:25)
Factual error: With a significant portion of Marcy's legs either shaved off her body or in a state of severe rot, it's doubtful she would even be able to stand up properly, let alone run away from a mad dog. And if she'd already lost as much blood as we later see smeared around the bathroom fixtures, she probably wouldn't even be conscious. (01:10:45 - 01:37:50)
Continuity mistake: When Marcy first runs out of the cabin, just before she dies, her chest is clean. But in the few seconds it takes her to run to the nearby toolshed, somehow her chest gets covered in splotches of blood. (01:10:45)
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)
Continuity mistake: Marcy made it all the way to the toolshed before the dog caught her and started mauling her, but her severed foot is later found only a few yards from the cabin. (01:11:20)
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 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 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)
Continuity mistake: In the scene where Paul and Marcy are having sex, you can see there is nothing on Paul's left wrist. The following shot of them Paul is moving his hands up her back and you can see he now has a watch on his left wrist.
Continuity mistake: When the rednecks are chasing the Blazer the Ford truck they're driving has round headlights. When the kid hits the deer and gets out the truck has square headlights and a different grill.
Answer: Sticking her finger up his bottom. I believe it is mentioned in one of the commentaries.