Oddly enough the two worst things I've ever done to a computer happened to the same computer 

  I was replacing an AT power supply on an old P166, and couldn't remember how the 4 wires connected to the case switch.  For some bizarre reason I figured I could just try a few different combinations and ended up being half blinded by a brilliant blue arc.  Thot I'd killed the power supply until I realized a circuit breaker had tripped.  Then I found out I 
had killed the power supply. 
Well anyway, after that I had to put in another power supply and I decided to actually connect it to the case switch to make sure I had the wiring right 

  I actually took the time to figure out where the wires should go and tried to connect them, but couldn't quite reach the switch terminals with my big thumbs.  So I decided to take the switch out of the case since it was only held by one screw.  I removed the screw and tried to remove the switch but it wouldn't budge.  Finally I ended up breaking the switch cuz I hadn't removed the button from the front of the case! 

  I just told the guy who owned the computer that he would need a new case 

.
Oh yeah, I've ghosted an image the wrong direction (target -> source), wiping out my only backup 

  In retrospect, I should probably give up my profession.