Well, I killed a K6-3 450 because of carelessness, not overclocking.
I was goofing around with my secondary system (the K6-3 450) and accidentally got the core jumpers and I/O jumpers backwards. In other words, I set my CPU core voltage to 3.3v instead of the 2.4v it should have been.
Ouch!
I know this was not what you were looking for, but it matched the post title, right?
The good new was the chip was under warranty, so I got a replacement for free! And I actually told the Tech Support/RMA department what I did! => He just laughed. . . .