Murphy's law is an adage in Western culture that broadly states "if anything can go wrong, it will." It is also cited as: "If there's more than one possible outcome of a job or task, and one of those outcomes will result in disaster or an undesirable consequence, then somebody will do it that way"; "Anything that can go wrong, will," the similar "Whatever can go wrong, will go wrong"; or, "Whatever can go wrong will go wrong, and at the worst possible time, in the worst possible way".

http://en.wikipedia.org/wiki/Murphy's_law