Usually
accidental, frequently
annoying.
Also rather easy to do, in my experience. I had suspected that I was one of the few people who managed to do this on a regular basis due to my somewhat unconventional computing setup
(PC lying sideways on the floor with no side panels or case - due to frequent need to fuck with its internals and the fact that it now contains six hard drives, a DVD Rom, a CDRW drive, and about half a dozen cards - something which makes it physically impossible to put the case back on anyway)
This configuration leaves the small and highly sensitive power switch lying approximately two centimeters in front of where my left foot normally resides when sitting comfortably. Thus is will not come as a surprise that reading a particularly amusing node, or playing a good game of Half Life will frequently cause a small foot spasm, resulting in the kind of rapid shut-down that Windows really doesn't like. I feel this may be a good idea because I am starting to suspect that all Windows PCs are programmed to email a tiny piece of your soul to Micro$oft every time you shut them down normally.
I wrote this node upon seeing a chat message from yossarian admitting he had just accidentally done the above to his PC, giving me a feeling of camaraderie with a fellow rapid shut-down brother.
P.S. I am considering disconnecting said power switch, mainly because I am a certified geek and almost never intentionally turn off my computer anyway...