I have long been a student of the above Law, but I have not seen it expressed so succinctly:
The law of unintended consequences is what happens when a simple system tries to regulate a complex system. The political system is simple, it operates with limited information (rational ignorance), short time horizons, low feedback, and poor and misaligned incentives. Society in contrast is a complex, evolving, high-feedback, incentive-driven system. When a simple system tries to regulate a complex system you often get unintended consequences.
That is a quote from Andrew Gelman, via a piece at Marginal Revolution inspired in part by a Dubner and Levitt (Freakonomics) piece in the NYT entitled The Red-Cockaded Woodpecker which explains how the Americans with Disabilities Act has resulted in lower employment levels among the disabled.
Both of the links are worth reading.
The cost of food around the world due to biofuels is a fine example. Michael Crighton's talk below does a fine job with the subject of complexity, when it comes to man's interventions in nature: