After the investigators have been given the knife by Driscoll, the knife moves around on the table between shots.
During the scene in which Alex and Mara are making love, the leather bracelet Alex wears changes position from one wrist to the other several times.
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 in exactly the same order as in English. Words that actually belong together can be placed at opposite ends of a sentence.
You can see the cable holding the man who was being hanged.
When Thomas is in the hospital speaking with Mara, the IV blood bag in the background is actually a zip lock bag.
The policeman tells Alex that Mara Williams has escaped. However, the character's name is Mara Sinclair.