Ill give you mine.
When I was in college I used to do a few nixers here and there. At the time I was studying IT in college and the was still in a time where most people didn't know how to fix their computers themselves.
I was asked to have a look at a computer that wouldn't start up properly. I wasn't entirely sure what was causing the problem but I had a look at the power supply unit. It has a little switch on it that said 240v and 120v, it was set to 240v so I thought, sure I flick it over and see if that does anything. So I flick the switch, plug it back in and turn the power switch on the PSU. Then it happened. Poof a big cloud of smoke came out of the back on the PSU. I quickly turned it off and put the switch back to 240V.
The owner didn't see what happened and when I was leaving I was saying I didn't fix it so I didn't want payment, but they insisted that even though I couldn't fix it that I still spend time on it. I took the £10 but I've never felt so guilty in my life.
Needless to say I never made that mistake again.
What are some mistakes you have made and will never repeat.
Oh and saying I put petrol in my diesel car won't count because my brother, the big idiot, has done that twice.