There was one time when my computer completely broke down, and I lost everything that wasn't backed-up. However, other than that, the worst error I've gotten...
One day I was browsing wincustomize.com, to see if I could find anything else to go along with my ThunderStorm theme. I was looking through the BootSkin library, and found a ThunderStorm theme. I dedcided to install BootSkin, along with the theme. I then restarted my computer to see how it would work.
When it started up, I saw the default Win XP loading screen replaced with the ThunderStorm one, but after a few seconds, a blue screen flashed on my screen, and my computer rebooted. Shocked, I went into Safe Mode. I then used System Restore, and after that, everything was fine.
So I guess the lesson is... never use ANY program that tampers with the way your computer boots up, even if it just changes the look of the loading screen.
HP Pavilion | Intel Pentium 4 CPU 2.40 GHz
512 MB Ram | NVIDIA GeForceFX 5600 AGP, with 256 MB Ram