I'm currently running an AMD K63+ 450 at 6x100=600 and 2.1 vcore on a Tyan 1598 Motherboard. This setup has been rock stable for a couple of months now and completely trouble free.
But last night all of a sudden the system crashed. When I tried to reset I got a blank screen and beep codes. So, I turned it off and opened the case, looking for anything getting hot. The CPU heat sink was so hot I could burn my fingers on it. so, after letting it cool off, I turned the system back on (just for a few seconds) with the case open and immediately saw that the CPU fan was not running. Inspection showed the fan was packed with cat hair!
So I fugured my CPU was probably fried, but just to be sure, I put a spare fan on there and tried it. To my surprise, the system now runs fine again and the CPU didn't seem to suffer any ill effects, at least in the short term.
I wish my wife were not so attached to those cats!