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 Bert returns from shooting, he talks to Jeff and Marcy, when returning his BB gun hangs over his right shoulder. Shot changes and his gun hangs over his left shoulder. When it cuts to the last back closeup of him and as he walks away to the cabin, you can see the gun has jumped to being over his right shoulder between the shots. (00:17:00)
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)
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)
Visible crew/equipment: At the start of the movie when the teens are heading off to the woods in the Jeep, in one of the backseat shots, you can clearly see a cameraman in the rearview mirror. (00:04:00)
Answer: Sticking her finger up his bottom. I believe it is mentioned in one of the commentaries.