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.
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.