I possibly fried a Radeon 5970...
I overclocked (under Linux) it to 800 MHz. It blanked after a few seconds of mining and the fan kicked in at max speed.
I (quickly) rebooted the computer and did what I thought would turn overclocking off: aticonfig --od-disable
After several seconds, I got a message announcing that overclocking was off. I relaunched poclbm but the hash rate was too high, meaning that it was still running at 800 Mhz.
Same scenario as before (black screen, loud fan). I was a bit slower to reboot. Probably too slow. A subtle but alarming smell came out of the card.
Now, the card doesn't POST and the fan runs constantly at max speed.
I've booted the machine with another graphic card. aticonfig
doesn't find anything. I'm afraid it's toast