Murphy’s Law (also known as "Sod's Law")
Anything that can go wrong, will.
Attributed to Captain Edward A. Murphy, USAF, 1949
Finagle’s Law
Anything that can go wrong, will—at the worst possible moment.
Larry Niven. "N-Space". 1980.
Fissure's Theory
If it involves anything believed to be so strong it won't break, count on it fracturing, rupturing, collapsing, or shattering.
Corollary One, The Not-In-My-Backyard Practicum
If someone is waiting, or you have a deadline, what goes wrong will be unexpected and/or obscure.
Corollary Two(a), The Pessimist's Postulate
If something involves technology, all bets are off. Troubleshooting will be unsuccessful in the amount of time available.
Corollary Two(b), Pediatrician’s Principle
Intermittent faults disappear when the repairman arrives. Can also be stated as "the kid is never sick when you take them to the doctor's office."
Corollary Three, The Optimist's Axiom
You know, you never know, you know.
Corollary Four, The Pragmatist's Benchmark
If nothing is wrong—something… is wrong.
See also Immutable Levels of Priority: Totally Scientific System of Things Getting Done