I learned something yesterday. In fact, I learned two things.
First, it’s a bad idea to install a desktop power supply unit in the dark.
Second, those keyed 4-pin ATX plugs will in fact fit quite easily in just about any vaguely ATX-shaped socket regardless of the orientation or configuration of the keyed pins.

What idiot designed the key-shapes on this fucking thing? A semi-octagonal peg will go quite easily into a square hole. And vice versa. FFS.
The good news is that my motherboard is smarter than the idiot who designed the crap ATX plug key system — and smarter than the idiot who jammed the fucking thing into the wrong hole too. Because despite a bad case of Voltage Abuse, in the cold light of dawn, once I’d literally seen the error of my ways and re-connected things properly, the machine powered up as good as new.
Phew!
Tags: abuse, ATX, computer, idiot proof, keyed pins, lucky, motherboard, power, PSU, voltage