Murphy's Law

Murphy's Law prov.  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 {luser}s.  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 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 "Anything
   that can go wrong, will"; this is sometimes referred to as
   {Finagle's Law}.  The memetic drift apparent in these mutants
   clearly demonstrates Murphy's Law acting on itself!