I screwed my overclock settings and my board wouldn't boot. Went to reset the CMOS memory but it wouldn't.
I shorted the battery jumper pins, no success. I removed the power cable, removed the battery, drained the caps by holding down the power button, left for 20 mins - no success.
Figured something must have died somehow. I systematically removed all the drives/cards/case connectors, video card, single memory DIMM. Same symptoms. Was looking like processor, board or PSU problem.
Finally figured it out - POWERED USB HUB! My keyboard is connected by a powered USB hub and must have been supplying current to the CMOS memory. I can replicate the problem by appying crap settings to make it fail, when I unplug the USB hub, short the battery jumper pins, it resets. If I don't unplug the USB, it doesnt.