I have found the following to be all too true. (BTW, I don't know if I came up with this law first, but I'll claim it until someone else does. Please notify me if you know of an earlier source.)

Bilén's Law of Conservation of Operable Systems: If n systems must be operable to perform an experiment, then at most n-1 will be operable at any given time.

Corollary: If you fix the last broken one, another will break to take its place.