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.
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.
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.
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.
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.
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.
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.
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.
Continuity mistake: When Marcy and Paul are talking about their hopeless situation "being like being on a plane that's going down" you can see a small wall hanging to the left of the glass panel door in the background. Also, the shadows from the hanging and door frame cast left, away from the door. Later, when they have sex in the same bedroom, the wall hanging is on the right side of the door and now its shadow casts towards the door. It's definitely not the opposite wall of the room, as this is shown earlier and has a wood-plank door, a much larger painting mounted on it and unlike the wall shown in these scenes, it isn't wallpapered.
Deliberate mistake: The sex scene between Paul and Marcy is broken up with a shot of Karen dying alone in the toolshed. The shot is well-lit, but we saw Marcy closing the door when she put Karen back in the shed, leaving the shed virtually pitch dark. So where is the light coming from?
Continuity mistake: When Paul and Marcy start having sex, Marcy's hair is running down her back. Paul asks Marcy about whether they should use a condom. Cut to a close-up of her face for her reply and her hair is suddenly hanging over the far side of her face. Cut back to the original angle and her hair is running down her back again.
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.
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.
Continuity mistake: Right at the end, when Paul checks on Karen (obvious dummy), she is lying on her right side while the dog is attacking her. After Paul gets chased out by the dog and kills it, he returns, and Karen is now on her left side. She didn't have the strength to move when the dog was biting her, so how could she turn over?
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"
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.
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.
Continuity mistake: When the kids are in the shop near the beginning of the film, at the shopkeepers desk is a cardboard box, and next to it are some jars and tins. Between a couple of shots the jars/tins rotate and move position on the desk, and also two or three jars/tins appear then disappear.
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.
Continuity mistake: When Marcy, Bert, Jeff and Paul are in the cabin talking about the fact that maybe they should prepare their own meals, during the conversation before the scrap between Jeff and Bert, watch Marcy who's sitting on the couch. Between shots she goes from having her hands up in the air, to folded or down in her lap.
Continuity mistake: When the blonde goes to kiss her boyfriend as they are floating on a platform in the lake, she rolls herself off the platform and completely submerges herself underwater. When she comes up for air, it cuts back to her boyfriend looking at her, then back to the blonde who is head above water and her hair is completely dry.