Murphy's law is a popular adage in Western culture, which roughly states that things will go wrong in a technical system. It is often roughly stated as "if anything can go wrong, it will", though this is in fact Finagle's law.
The original Murphy's law reads:
Edward A. Murphy, Jr. was one of the engineers on the rocket-sled experiments that were done by the U.S. Air Force in 1949 to test human acceleration tolerances (USAF project MX981). One experiment involved a set of 16 accelerometers mounted to different parts of the subject's body. There were two ways each sensor could be glued to its mount, and somebody methodically installed all 16 the wrong way around. Murphy then made the original form of his pronouncement, which the test subject (Major John Paul Stapp) quoted at a news conference a few days later.
Within months "Murphy's law" had spread to various technical cultures connected to aerospace engineering. Before too many years had gone by, variants had passed into the popular imagination, changing as they went. Most of these are variants on "If anything can go wrong, it will"; this is sometimes referred to as Finagle's law or Sod's law. Another well known application is to household probability: "The chance of a dropped slice of bread landing buttered-side down on a new carpet is proportional to the price of the carpet."
O'Toole's commentary on Murphy's law was : "Murphy was an optimist!"
These mutant versions demonstrate Murphy's law acting on itself - or perhaps Finagle's law acting on Murphy's law.
See also: Finagle's law, Sod's law, Hanlon's razor, Sturgeon's law, Law of Infernal Dynamics