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)
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: 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)
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.
Continuity mistake: When Bert first realises he has the fever, he crouches beside the car and starts coughing up blood. When he hears the others coming he wipes his mouth, leaving a small trail of blood on his chin, but when he stands up and talks to them the blood is gone and his chin is clean. (00:54:55)
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)
Continuity mistake: When Deputy Winston is talking to Paul, he takes his tinted glasses off, in his left hand. Then in the next shot he has them in his right hand, although he never swapped hands. (00:38:05)
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)
Factual error: A doctor is seen using a syringe to inoculate the individual with the disease. It is clear that the containment gloves and sleeves he is using are not proper as you can see his skin between the gloves and the sleeve. If this were a real containment unit his entire skin would be covered. (00:31:00)
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)
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)
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 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)
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)
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)
Answer: Sticking her finger up his bottom. I believe it is mentioned in one of the commentaries.