This isn't something stupid I did, But my girlfriend did.
I had an overclocked 6800LE, It was so beautiful.
Anyways, My computer chair was broken, So my girlfriend start pulling the Couch up to the computer in-order to carry on using it. Which was fine.
Now I have an Antec Sonata II piano black case, With 2 vents, 1 on front and 1 on back.
My Back fan is my intake and the front fan is the exhaust.
We leave the PC on over night downloading, Always do.
This particular night, I set the downloads off while my girlfriend is on the computer, I wake up the next morning, To find the couch is still right up to the side of the computer blocking the exhaust, my girlfriend hadn't moved it back, And it's not turned on... FEAR STRUCK.
I moved the Couch back, and tried everything to get the system back online. But unfortunately, My girlfriend had essentially turned my computer into a fan assisted oven, and she cooked my Graphics card.
For those who don't get it, Air could get into my computer, But not escape due to the blocked exhaust vent. That's a bad environment for overclocked components.
Thankfully my overclocked Opteron was a little tougher than my Graphics card.