We are told that Nicodemus used Barabbas' curse on Harry, a curse that kills his target with no chance of escape. Judging by the events of the book, however, the curse doesn't work by directly targetting him (like the entropy curse), but arrainging the events in such a way they'll lead to the target's death. Shiro also tells us that, while the curse can not be broken, the Knights can redirect it on themselves, if they chose to, and that he did it for Harry. That's why the Villain Ball strikes Nicodemus: he shouldn't have accepted the deal, but the very same curse he cast to ensure Harry's demise was altered, so that Shiro had to die in Harry's place, and that's exactly what happened.
When one of the Swords of the Cross is used to cut Harry's ankle free, it also nicks him a little bit by accident. Despite the fact that it's not a very bad cut, it very clearly hurts like hell - way more than it ought to, and enough for several people to tease him about his reaction to it. Come book 17, Battle Ground, Harry's Roaring Rampage of Revenge is stopped by cutting it out of him - with Fidelacchius, in a Contrived Coincidence . Talk about foreshadowing...
