Link: http://wgsf.oldgleaner.com/

A very necessary and oft-quoted axiom - an accepted truth, dictum, truism, principle; maxim, adage, aphorism - at WGSF Television was Murphy's Law.
Simply put, it states:

If anything can go wrong, it will . . . At the worst possible moment!

There is a corollary known as IPIO - The Innate Perversity of Inanimate Objects.

That is the nature of the final screw to fall into the device being reassembled. Or, a slice of bread to fall buttered-side down.
The 'watched pot' that doesn't boil, or the printer/copier that can sense that you are in a hurry, and jams!
IPIO