Plot hole: Every ghost that we see in the house looks the way they did when they died, for example the girl from the film's intro is seen without skin on her face, which happened to her before she died. However, in the final scene when we see Simon's ghost joining them, he looks completely normal, when he should have had a throat slit wound and the carved markings on his skin. (01:31:20)
Factual error: The credits list several of the characters as "Arab." But the film is set in Afghanistan and the Afghans are not Arabs.
Continuity mistake: When she gets the pizza from the guy at the door, it is in a white box. When she goes in and sets it on the counter, it is in a brown pizza box. Then later, when she is running away covered in blood, she grabs the knife on top of the pizza box, and it is white again.
Revealing mistake: After the 4 have been tied up and babyface has killed their captors, he grabs Alexa by the neck. There is a shot of him from behind and you can see the make up covering the back of his head/neck coming up above his collar.
Character mistake: At the hospital, the inspector manages to get from the nurse the file of the patient with the liver condition. Forgetting the fact that the Turin, Italy hospital is using English for its documents (it's simply a cinematic convention), the problem is that the letterhead of this document says "Department of Chronic Deseases", misspelling the word.