If something can go wrong, it will.
is 60 years old this year.
http://www.guardian.co.uk/science/2009/jan/05/murphys-law-accidents has a nice summary of the history. Some of the key parts:
we now know it is named after Edward A Murphy Jr, a test engineer for the McDonnell Douglas aerospace manufacturer ...
One experiment apparently involved a set of 16 sensors attached to the subject's body. These could be mounted in one of two ways, and one of Murphy's assistants installed all of them the wrong way round, resulting in a zero reading. According to Robert Murphy, Edward's son, the words his father uttered at the time were along the lines of: "If there's more than one way to do a job, and one of those ways will result in disaster, then somebody will do it that way."
This we might term the original Murphy's Law. However George Nichols, another engineer present at the experiment, recalls the phrase as: "If that guy has any way of making a mistake, he will," a rather cruel jibe later more kindly condensed by the McDonnell Douglas team to: "If it can happen, it will happen." Major John Paul Stapp, the subject of the experiment, then reportedly summed up the newly coined law at a press conference some days later as: "Anything that can go wrong, will go wrong."