Deliberate mistake: There is one scene in the movie where Heath Ledger reads a Latin sentence in a book. As he is translating the sentence, he goes over the words one by one with his finger. This would never be possible in a Latin sentence, because words in Latin are never - I repeat: never - in exactly the same order as in English. Words that actually belong together can be placed on opposite ends of a sentence.
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.
New York Detective: Is says here she tried to kill you at an - exercise class?
Alex Bernier: She tried to kill me at an exorCISM.
Alex Bernier: And now it is I. I have been blessed and cursed... for now I possess the keys to the kingdom of heaven. I will forgive those who deserve freedom. I will damn those who have damned themselves. I will learn to live after love has died. I am the sin eater.