My knowledge of the Dark Brotherhood is murky at best, and having played through their quest lines in both Oblivion and Skyrim, I'm curious as to how the Dark Brotherhood handles contract failure. What happens to the assassin when he/she fails a contract? Additionally, what happens to the contract target? Does the Dark Brotherhood select another agent or do they let the contract go?

Multiple people try to assassinate me in one play-through, so I suppose they send other assassins, and failed contracts would most likely end up with the assassin sleeping with the fishes...

I imagine that most failures would be the result of an assassin having been killed. They would send another assassin after the target, however. The contract has to be completable, or else they likely wouldn't accept it in the first place.