The correct, original Murphy's Law reads:
If there are two or more ways to do something, and one of those ways
can result in a catastrophe, then someone will do it.
This is a
principle of defensive design, cited here because it is usually given in
mutant forms less descriptive of the challenges of design for
lusers. For example, you don't make a two-pin plug
symmetrical and then label it THIS WAY UP
; if it matters
which way it is plugged in, then you make the design asymmetrical (see also
the anecdote under magic smoke).
Edward A. Murphy, Jr. was one of McDonnell-Douglas's quality-control
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 in a
replacement set the wrong way around. Murphy then made the original form
of his pronouncement, which the test subject (Major John Paul Stapp)
mis-quoted (apparently in the more general form Whatever can go
wrong, will go wrong)
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 Anything that
can go wrong, will
; this is more correctly referred to as
Finagle's Law. The memetic drift apparent in these
mutants clearly demonstrates Murphy's Law acting on itself!