Murphy's law

According to Wikipedia...

Murphy's law is an adage in Western culture that broadly states that things will go wrong in any given situation, if you give them a chance. "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." It is most often cited as "Whatever can go wrong, will go wrong" (or, alternately, "Whatever can go wrong will go wrong, and at the worst possible time, in the worst possible way" or, "Anything that can go wrong, will," or even, "If anything can go wrong, it will, and usually at the most inopportune moment").