Finagle's law
Finagle's law of dynamic negatives (also known as Melody's law, Sod's Law or Finagle's corollary to Murphy's law) is usually rendered as "Anything that can go wrong, will—at the worst possible moment."
The term "Finagle's law" is often associated with John W. Campbell Jr., the influential editor of Astounding Science Fiction (later Analog).