Overclocking too high and trying to run a game when people don't know what they are doing can damage a card. Usually sometimes undoing the overclock can bring everything back to normal, but sometimes it goes too far and it's damaged like that in an instant. Some people think oh, the slider goes up to 5000mhz, so lets crank it that high and hit apply.
Rivatuner lets you fiddle with bios level things and probably trying to unlock shaders and all that hacky type things if I recall, so doing that you can certainly damage it. Write the wrong file to a $1200 GPU that doesn't have dual bios' and poof.