Andrew Gelman wonders about the Law of Unintended Consequences: "Stephen Dubner and Steven Levitt wrote this Freakanomics column, which concludes, "if there is any law more powerful than the ones constructed in a place like Washington, it is the law of unintended consequences." What I'm wondering is, what sort of law is this? Obviously it's not a real "law" like the law of gravity or even one of those social-science laws like Gresham's law or the statement that democracies usually don't fight each other. But it's supposed to be more than just a joke in the manner of Murphy's law, right?"
I think the Law of Unintended Consequences can be seen as a social science / organizational behavior law.
First, let's get a definition, which I'll borrow from http://www.fredlaw.com/articles/health/heal_95su_kjf.html "the proposition that [MOST] every undertaking, however well-intentioned, is generally accompanied by [GENERALLY] unforeseen repercussions that CAN overshadow the principal endeavor."
I added "MOST" and [GENERALLY] to the definition above to tone it down a bit.
Another variation on this is to look at a list of the effects of a proposed policy, and to realize that there are probably more effects than the ones listed. In fact, at heart it's a lot like forecasting error, particularly the forecasting error caused by unforeseen events outside the model.
I think as social science goes, its a decent law. It's even testable. For example, we might take old environmental impact statements from some years ago and see how well they anticipated what we would now regard as the true impact of a project.