Unlike the Laws of Gravity and Thermodynamics, the Law of Unintended Consequences has never been formally defined. That said, everybody has a pretty solid notion of it. It might be loosely stated like this: Any action is certain to have results not foreseen, that may be contrary to, or mitigating of, your intent.
Paul, to kill two for one, I'll republish this from the Greanville Post repost.
Good one!
Jeff