I had a K6-233 which was always on the verge of overheating (or maybe I was obsessing a bit too much about monitoring it). I envied my friend's K6-200, which was more stable it seemed.
My K6-233 got so hot it demagnetized its CPU fan. I noticed after the floating point unit fried the first time I tried to play some post-quake FPS.
After that, it would boot, but the antivirus software I was running said it had a virus. I guess that was the only msdos program in the boot sequence that performed floating point.
I had a K6-II which I put a big heatsink and large fan on (had to ghetto mod the mounting), sawed a 120mm hole in the case just above the cpu and put another fan there. It overclocked handsomely and stable after that. I don't remember the exact frequencies but I think it was 266MHz stock and clocked close to 400MHz with some modifications.