The vacuum cleaner done it.
Early 1970s, testing a new mainframe for ICL. Intermittent power-offs - nothing new there except they always seemed to occur at around 6 o'clock at night (this was before we had 18:00). I noticed that it was when the cleaner person moved the sucky machine next to the CPU cabinet. Tests ensued, and sure enough, that was the cause.
For several years thereafter we had to perform a standard vacuum cleaner test on every new machine.
Biting the hand that feeds IT
