Murphy's Law (common version): Anything that can go wrong, will go wrong.
" The modern version of Murphy's Law has its roots in the US Air Force studies performed in 1949 on the effects of rapid deceleration on pilots. Volunteers were strapped on a rocket-propelled sled, and their condition was monitored as the sled was brought to an abrupt halt. The monitoring was done by electrodes fitted to a harness designed by Captain Edward A. Murphy.
After what had seemed to be a flawless test run one day, the harness's failure to record any data puzzled technicians. Murphy discovered that every one of its electrodes had been wired incorrectly, prompting him to declare: " If there are two or more ways of doing something, and one of them can lead to catastrophe, then someone will do it".
At a subsequent press conference, Murphy's rueful observation was presented by the project engineers as an excellent working assumption in safety-critical engineering. But before long- and to Murphy's chagrin- his principle had been transformed into an apparently flippant statement about the cussedness of everyday events. Ironically, by losing control over his original meaning, Murphy thus became the first victim of his eponymous law."
Excerpt from Scientific American, April 1997, "The Science of Murphy's Law" by Robert A.J. Matthews