Reboot to POST
Four servers (actually desktop-class PCs) which lived on a bank of industrial shelving -- steel, bolt-together, Meccano-set-looking stuff. There was a shelf at sit-down working height that held the row of four keyboard/monochrome-monitor pairs (no mice; this was all strictly text mode). The actual computers were on the next shelf up.
They were running Xenix, which wasn't terribly stable, so crashes were a fact of life, but the rightmost of the four systems had a different failure mode. Instead of a Xenix crash's typical "Hit any key to reboot", it would snap right to POST. System load was the usual culprit, but this was the backup machine and so pretty much idle during work hours. No rhyme or reason to it...
... until I was doing some work on #3 next to it, hit Enter on the command to put the task I'd been working on into effect, and #4 instantly did its snap-to-POST thing...
... and it finally penetrated my hyperfocused-on-software brain that (a) it couldn't possibly have been the fault of the command I'd just typed, seeing as it was a different box that crashed, and (b) that sequence of events -- type a command, #4 crashes -- felt vaguely familiar.
Having finally noticed the pattern, it was a matter of seconds to isolate the problem. (Given the context, I'm sure you've all guessed where this is going.) #4's power cable was loose in its IEC socket. It was fully seated, but even so, it wiggled a bit; and a vigorous Enter-key hit (as one does to "make it so" on a long bout of work) was enough to propagate through the shelves and wiggle it.
One decapitated (to prevent reuse) and replaced power cable later, and problem solved. Fortunately it was the cable and not the socket that was faulty.
[A "done and dusted!" icon would not be amiss.]