When Bert is shooting in the wood, the beer bottle he is drinking from is clearly empty. When he puts the beer bottle down by the fire, it changes briefly into a green wine bottle, and then back into a beer bottle again in the next shot.
In the scene where Marcy is attacked by the dog, the camera shows her trying to get into the shed, and then cuts to the dog. When the camera shows her being taken down by the dog, she is wearing blue jeans. This is because the dog used was so vicious, the actors could not be around it, so a guy had to step in with protective layering on his arm to be attacked.
At the beginning of the sex scene between Marcy and Paul, Paul's left forearm is clearly seen on the pillow and he is not wearing a watch. A few seconds later, when his hands are on Marcy's back, there is a watch on his left wrist.
When Marcy notices the red hand-shaped marks that Paul left on her back, the fingers are pointing inward towards her spine, but the thumbs turned outwards, towards her armpits. Later, when the marks develop in to a pattern of weeping wounds, the thumbs have changed position, pointing parallel to the other fingers.
After Deputy Winston takes Paul's statement, he puts his note pad in his pocket with the pen facing outward to the left. In the next shot, the pen is facing to the right and in the shot after that the pen disappears completely for one shot before reappearing and facing the right.
After Paul disinfects himself following his quick fling with Marcy, the level of Listerine in the bottle is clearly beneath the word "Listerine" on the label. In the next scene, when Marcy is washing her face, the bottle is clearly visible in the background and the level in the bottle is clearly above the word "Listerine".
After Jeff runs away from the Cabin with the beer supply, Paul and Marcy drag Karen back in to the tool shed and close the door, leaving her in complete darkness. Minutes later, while Paul and Marcy are having sex, Karen is shown to be laying in a well-lit tool shed. The shed has no windows, so apparently the door has been mysteriously re-opened.
After washing her face in the washbasin, Marcy turns around to examine her back in the mirror, while clutching a hand towel. As she does so, the shot switches to a close-up. There is much more of the towel fanning out above her hands before the close-up than after.
In scenes immediately before and after she sleeps with Paul, Marcy is wearing a silver ring on her right hand, yet it seems to disappear during the sex scene. Considering how impulsive the act was, it is unlike she would have stopped to remove it, and then replaced it.
When Paul and Marcy first start having sex, the camera shot is looking over Marcy's shoulder. When Paul asks her, "You don't use condoms?" the bulk of Marcy's hair can be seen cascading down her back. Then the camera angle changes to a close-up of Marcy's face for her immediate reply, "Don't worry, I'm healthy." In this shot, most of Marcy's hair is suddenly hanging over the left side of her face.
When Paul uses the bathroom after having sex with Marcy, there is a blue cylinder (it appears to be an aerosol) resting on top of the toilet cistern. Then, in the next scene, when Marcy is in the bathroom washing her face, the cylinder has suddenly shifted to atop the green shelves above the toilet.
The bedclothes for Marcy and Jeff's bed remains consistent for the duration of the movie, save for the light-colored bed-sheet that appears during the sex scene between Marcy and Paul, that covers Marcy's behind, and then is not seen again.
Facing inwards from the door, there is a rusty old muffler seen several times in the tool shed, on the right-hand-side. However, during the brief clip of Karen in the shed that occurs during the sex scene between Paul and Marcy, the muffler has moved to the left-hand-side of the shed.
Through the course of the movie it is established that the door to the immediate left of the bathroom is Marcy and Jeff's bedroom. During the sex scene between Marcy and Paul this bedroom is well lit, but in a wide shot of Paul entering the bathroom immediately afterwards, we see that the room is in darkness.
When Marcy gives her pessimistic "Like Being on a Plane" speech, the door frame and the wall hanging behind her cast strong shadows. Moments later, when she begins having sex with Paul, not only are the shadows much lighter, they are also cast in the opposite direction.
At the beginning of the sex scene between Paul and Marcy, the red pillow on the left side of the bed is virtually upright, pinned there by Paul's head and the white pillow. Towards the end of the sex scene it is suddenly laying flat on the bed.
The scratch marks/rashes Paul leaves on Marcy's back while they're having sex should develop fastest at the fingertips (where the most pressure would've been applied) and slowest at the palms. This is the case during the part of the sex scene where Paul actually makes the marks, and later when the rashes begin to develop into weeping wounds, however in the scene in between, where Marcy first notices the rashes, their color is uniform.
During the first sex scene, Marcy is wearing small earrings, while in all the scenes leading up to it, she is wearing large hoop earrings. This is because Cerina Vincent shot both her sex scenes back-to-back and her character was wearing smaller earrings at the time of the second sex scene.
During the scene where Karen begins vomiting blood, there's a shot where you can see down Marcy's jeans and see that she is wearing white or light blue panties. However, in the scene where she is washing her face in the bathroom shortly after-wards, she is wearing panties with a leopard-skin design.
In the quick dog's-eye-view shot of the mad dog attacking Marcy, Marcy is wearing both her sandals, however, this is followed by a blurry wide-shot of the dog savaging Marcy, with one of her sandals in the foreground, many feet away from Marcy.
The sex scene between Marcy and Jeff is divided in to two parts by an intervening shot of Bert peeing in a creek. Before the break, the white pillow is seen beneath Marcy's head. But immediately after the break, despite the fact that Marcy and Jeff have virtually not moved, the pillow can't be seen and is later revealed to have moved all the way down behind Marcy's back.
When Marcy is sitting in the bath shaving her legs the wounds on her back are severe, but not bleeding. A moment later, when Marcy starts panicking because the flesh is peeling off her legs, the area all around her back wounds is suddenly smeared with a large amount of blood.
When Marcy first starts having sex with Paul, the left side of her is illuminated and the right side is in shadow. But when the camera angle changes to being directly behind her back, the right side of her body is illuminated and the left side is in shadow.
When Marcy first jumps up in the bath after shaving her flesh off, there is clearly still a fair amount of shaving cream on her legs. But a split second later, in the close-up of her bleeding legs there is no trace of shaving cream to be seen.
The last time we see the sores on Marcy's back, the finger marks on the left hand print are pointing virtually upwards, towards her head. When Paul was making the marks while having sex with Marcy, his hands are never at such a steep angle.
When Bert returns after accidentally shooting the woodsman, Marcy and Jeff yell at him for leaving the fire unattended around the campsite; as he walks past Marcy and Jeff, he has a cigarette in his mouth, but when he turns around and the camera angle changes, his cigarette goes from being in his mouth to in his hand.
There is a cameraman visible in rear view mirror at the beginning of the film as the characters begin their trip out of town on the way to the cabin. Although this should normally qualify as a goof, this scene was left in by director Eli Roth on purpose. See also trivia section.
Paul tells Marcy that he's going to get Jeff so that the three of them can walk out of the woods together. But when Paul races back to the cabin after his short journey, he runs in from the left side of the cabin. Paul knew that Jeff went off to the right when he ran away.
The water supply in the cabin was connected directly to the reservoir. Water from reservoirs cannot be used for domestic purposes without a large treatment plant to remove minerals, kill bacteria and perform general filtering. Using a reservoir as a water supply for a remote cabin is highly improbable.
Though the scenery dictates fall, and the characters say that they had just finished finals, they mean that they had just finished their Fall semester finals, not their end of year or spring semester finals.
Justin (Grim) is clearly seen tying his dog to a tree when he meets the main characters at the bonfire. It was thought that when he leaves he just calls his dog and walks away, however you can still see him crouching by his tied up dog as the camera pans away.
Paul told Marcy that he was just going to get Jeff and then they (Paul, Jeff and Marcy) were going to walk back to town together. So why does Marcy draw a bath and start shaving her legs instead of preparing for the journey?
When Marcy starts having sex with Paul, despite the fact that she throws him down on the bed from an upright position, when she falls on him there is a bed-sheet covering her ass. It's obvious that the sheet has somehow been stuck to her body, as it would have fallen off otherwise. (See trivia.)
In the scene where Marcy is lamenting that they are all doomed, in the background, there is a curtained window/door on the RIGHT hand side, and a oval-shaped wall hanging with a white frame on the LEFT. A few seconds later we see a close-up of Marcy's face and in the background, the wall hanging is on the right and the door on the left. Quite obviously, one of these pieces of footage was horizontally flipped.
When Jeff discovers the bathroom in it's final, bloody state. There is blood in places that Marcy (the source of the blood) would never have touched, such as high up on the column of the bathroom sink.
When Marcy is running from the vicious dog, you can see that she is wearing something white within the sleeves of her robe. She has just left the shower in a panic, so it is unlikely she would've put anything on before the robe.
After the sex scene between Paul and Marcy, Paul goes in to the bathroom to disinfect his privates. Well before he reaches the bathroom, but a split second after the scene begins, the bathroom suddenly lights up all by itself.
When Paul wraps his arm around Marcy's back after they finish having sex, the base of his arm is about level with Marcy's waist. From this, it is obvious that Marcy was gyrating on Paul's chest or stomach, not his crotch.
After Marcy flips on top of Jeff while having sex with him, there are a few brief instances where you can see a red cord/string flailing in the background between their bodies. This is the tie-string from some piece of clothing (most likely a bikini bottom) that she was obviously wearing.
In the scene where Marcy is sitting on her bed, chastising Paul for "doing a number on her back", the bed frame which passes in front of the window, casts no shadow on the bed. This proves that the external "daylight" is really artificial light, as true sunlight is much brighter than any light artificial light that would've been in the cabin.
The interior design of the cabin is unusually uncoordinated. Some walls are a messy assembly of planks on one side, while wallpapered on the other, despite the fact that the room has a similar plank design on another wall, etc.
Just before Marcy first throws Paul down on the bed to have sex with him, there is a deep dent in the white pillow the exact size and shape of Rider Strong's head in the exact position where his head lands. Obviously this dent was left over from a previous take.