Continuity mistake: Towards the end, right after Mia crawls through the tunnel and into the shed, she backs up against some shelves. There is a lot of blood on her face and her hair is hanging over her face. The shot changes to a close-up and her face now has much less blood on it and the hair that was hanging in her face is gone. (01:16:25)
Factual error: Near the end, Mia discovers the chainsaw in the toolshed. Glancing around, she sees a clear plastic bottle of sparkling bright blue fluid that looks like window cleaner but is marked "Chainsaw Gas." Real chainsaw fuel is a 1:40 mixture of 2-cycle motor oil (greenish-black in color) and gasoline (pinkish-amber in color), which produces a fuel that is olive-green in broad daylight. In the darkened shed where Mia finds the chainsaw, the "Chainsaw Gas" in the bottle should not appear sparkling blue, but should appear very dark green, even with artificial light behind it.
Plot hole: The book says the demon has to devour 5 souls in order for the abomination to return, but it only got 4 since David saved Mia's.
Suggested correction: In the extended version, we see Mia's soul isn't actually saved.
"In the extended version" has no relevance. Bonus content holds no weight when leveling criticism at the theatrical cut of a film.
Suggested correction: Actually, more than five souls were claimed over the course of the film, whether or not you count Mia. Four people died in the cabin, plus the possessed girl in the beginning of the film, in addition to the girl's mother, who is mentioned to have been killed by her. You could also make an argument that the dog counted, technically. You also gotta remember, the book said the souls must be "consumed," and technically, it did possess Mia and she "died" while still possessed before being revived. So by the vague definition of "consumed," she could still possibly count. Either way, definitely more than five souls were claimed over the course of the film.
Brother and friend were also technically cleansed by fire.
Continuity mistake: When Eric locks Mia in the basement after she vomits all over Olivia's face, the amount of vomit on Olivia's forehead drastically changes between shots.
Revealing mistake: When the teenager is kidnapped at the beginning of the movie, the gun used to knock her out never makes contact with her face.
Continuity mistake: During the middle of the movie Mia rips off the buckwood necklace that her brother gave her at the beginning. She snaps and breaks it, but later in the film it is seen in one piece, and at the end she puts it on in an emotional expression of remembrance.
Audio problem: When Mia tries to escape in the station wagon there is the sound of tyres squealing on asphalt when the car is on mud.
