When I was around 12, I was learning about overclocking, and accidentally killed my dad's graphic card, an Nvidia FX 5900.

I vividly remember launching The Sims 2 to test my overclock, when suddenly the screen started turning on and off (the video driver was probably crashing and restarting), and after I reset the PC, there were 2 green lines on the screen and XP was stuck in 640x480 16 colors because not even the basic display driver was able to load.

My dad was mad obviously because it was an expensive card, the damage wasn't covered by the warranty, and he was into gaming too at the time. I was stuck with integrated graphics for about a month while we waited for the geforce 6000 series to come out.

I was so scared of overclocking after this happened, I didn't try it again until a few years later years later when I had my own computer (and killed another card, a 9800GX2).