Ooh boy I've got a dozen but I'll post here a (shortened down) version of my most "weird" encounter:
It begun when I wanted to pop in a P233MMX into my Shuttle HOT-555A board (rather affectuonously known as "Shittle Hot garbage"), set the jumpers correctly and booted it first try, or so I thought...
It would post fine but would flake out when trying to start Windows, popping Scanreg in a constant loop.
Bemused, I attempted to reinstall Windows, no dice. Got a Suwin.exe error, tried with w95, same deal.
Then I get a HDD error, immediately I swap the 20GB WD IDE HDD I had there at the time (gah why did I smash it to bits... My own stupidity will never cease to amaze me), put a SCSI IBM DDRS HDD (aka "Ultrastar 9ES"), with a Adaptec AHA-2940U2W, same punishment...
So I popped back my P133, set the jumpers accordingly to the manual, and it works... Though later on the WD drive started acting dicey so I left the 9ES HDD in there.
Now, if the PSU is dying, I'm fucked. If the 233MMX I got is a dud, I'm... Well, that blows. I'll maybe get one of those industrial ATX PSUs that still bear a -5V rail and a ATX2AT harness (I'm kinda bummed as I wanted to keep this PC as original as possible).
I even tried putting a fan on the voltage regulators, running the mobo outside of the case (never again, it's a plaie sans nom to tear down, and in the process I managed to give myself a few small cuts on my fingers, which were 20x more painful than papercuts, but since old PC cases sometimes require a blood sacrifice...)
Proud owner of a Shuttle HOT-555A 430VX motherboard and two wonderful retro laptops, namely a Compaq Armada 1700 [nonfunctional] and a HP Omnibook XE3-GC [fully working :p]