Continuity mistake: In the scene where Heath Ledger and his priest friend receive the dagger from the head of the Order that supposedly kills the Sin Eater, the head priest sets the dagger on the table between the 2 priests with the handle of the dagger facing Heath. The shot then moves from one side of the table to the other side showing different angles of the conversation, in each shot the dagger is no longer facing Heath but the other priest. But in the next shot is is, and so on for about 8 shots.
Continuity mistake: When Thomas follows the demon who is taunting him down in the sewers, there is a shot of him with a cut and blood on his face/nose. The camera changes to the demon and then back to Thomas, and the cut and blood are gone. Later, in the hospital, he has the cut back on his nose.
Continuity mistake: Near the end, when Alex finds Mara dying, her wrists slashed, an overhead shot shows that Mara's dress is spotless. The camera cuts to another angle as Alex kneels over her body, and now her dress is splattered with blood all across her torso.