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.
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.
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.
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.
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.
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.