I came across this interesting one today: Jevons paradox. Briefly stated, it is that more efficient use of a resource will often cause its consumption to increase.

Clearly it is just a restatement of a simpler principle from economics — cheaper goods sell more — but stated in this way it is a little new and clever, and certainly gives an interesting rebuttal to the idea of energy efficiency as a way to solve the world's energy crisis. Just as government intervention in Britain to reform the car industry caused the price of cars to fall, and hence caused car use to rise, in direct opposition to their election promises to cut car use, the law of unintended consequences and Jevons paradox may be the death of energy efficiency as a silver bullet.