Several years ago, a colleague brought to me a PC that he'd built for himself - I'd acquired a reputation within the company of being someone who'd fix/upgrade PCs for little more than some chocolate or cider in recompense.
Anyway, this PC had two problems:
1) it wouldn't power on, at all, and
2) the chassis was faulty - must be, because the ATX I/O shield wouldn't line up with the rear aperture and couldn't be installed.
Some of you can maybe see where this is going.
The owner had carefully assembled the PC, but - seeing no need for the small brass standoffs - had methodically screwed the motherboard straight to the base plane of the chassis. So of course it was shorting out in about a hundred places.
I removed it, and re-installed it with the appropriate standoffs - and, mirabile dictu, it fired straight up, right as rain.
As a bonus, I was suddenly able to install the I/O shield at the same time. Funny, that.
But what was even funnier is that the person in question worked for our IT department in a quite senior helpdesk services role. Obviously a software guy and not hardware!