Technically, the full Murphy's Law is "if there are two or more ways to do something, and one of those ways can result in a catastrophe, then someone will do it." The version that most people use is sometimes referred to as Finagle's Law.
The original was made, if I remember correctly, in response to a pin in some sort of US Air Force vehicle that could be inserted in two ways: the way that worked, and the way that caused huge problems at approximately the worst time possible. It's essentially about defensive design, in which the only ways in which to assemble or use something function correctly, like making the aforementioned pin only go in one way.
http://m.funsubstance.com/fun/413413/murphys-law-and-my-fridge-in-the-bg/
The original was made, if I remember correctly, in response to a pin in some sort of US Air Force vehicle that could be inserted in two ways: the way that worked, and the way that caused huge problems at approximately the worst time possible. It's essentially about defensive design, in which the only ways in which to assemble or use something function correctly, like making the aforementioned pin only go in one way.