I looked up porn on a school computer.
Now for a serious reply: the most illegal thing I have ever done was not with a computer but with (you guessed it) electricity/electronics. In the early days when I was exploring the fundamental techniques of radio, I assembled a powerful transmitter without thinking twice. I apparently created a little scrambling in the firefighter/PD radio. Am too afraid to play with radio waves outside of a Faraday's cache ever since.
This is not a "confession" because it was an accident, but technically it is illegal.
Of course, this accident is not related to 0x00sec. Mods can edit this reply as they please.