The double barrel shotgun keeps changing. It goes from having a pistol grip stock to a full stock.
When Eric goes to check on Olivia, he walks into the room and looks toward the bathroom door. As you can see the door knob is on the right side of the door, but when he goes to open the door, Camera shot from the floor, the door knob is on the left.
In the first half of the movie Mia severely scalds herself with boiling water leaving large blisters and burns on her face. These marks disappear without explanation in the film's finale.
After Mia frees herself from the car wreck and gets up, the cabin is visible behind her and it is still engulfed in flames despite the fact that at that point it had been raining for a while which should have extinguished the fire.
At the beginning of the film a woman is speaking a foreign language, according to the subtitles she is speaking Turkish, she is actually speaking in Welsh.
When David tries to resuscitate Mia he leaves on the plastic bag over her head.
After Mia crashes the car into the pond, the car has its footwells full of water. The doors are above the water line and at such an angle that it could not flood as shown. The car's depth in the pond is also inconsistent with the area immediately around the car; Mia exits the car and into the water at a depth near her waist.
Fibrillation of the heart is rare in young adults, whether from an OD or from suffocation. The first time Mia is mentioned as being defibrillated she was "legally dead", but legally dead people would not be defibrillated. The second time, the makeshift setup for defibrillation would only damage the heart, was placed incorrectly to defibrillate anyway, and would likely be ineffective in the case of suffocation.
Nailers have a safety feature that prevent them from firing unless they are pressed against a workpiece. Using a nailer as a weapon as shown in the movie wouldn't be possible unless the tool had been specifically altered.
When Mia is underneath the pick up truck trying to work the chainsaw, she has cotton wool in her right ear.
When arriving at the cabin Mia is complaining about a really bad smell, which the others only recognize after the cellar door is opened. But since the floor is made out of wooden plates with visible gaps between then, the smell should have been noticed by everybody from the beginning.
The closeups of the burnt cats in the cellar reveal that they're simply stuffed props.
Jane Levy's natural blonde roots are visible whenever she is in bright light.
When Mia tries to escape in the station wagon there is the sound of tires squealing on asphalt when the car is on mud.
In the beginning of the movie, Mia makes David cross his heart. He makes the motion over opposite side of his body (his right side).