Man I do stupid stuff all the time. I probably shouldn't even be getting into this hobby, it's going to be a matter of hours until I electrocute myself, or blow up my car.
As a kid, we had our boy scout meetings in an old church. After the meeting, my friend and I would go hang out in the basement and b.s. for a while. Anyways, I saw a fuse box (didn't know what it was at the time) with the oldschool large glass fuses in it. I thought they looked cool, and tried to take one out. I still remember that day lol.
About 2 years ago, I was doing work on my bike's carb. After I was done, it wouldn't start. Took me 2 hours of assembling and reassembling the carb, fuel lines, gas tank, starter, etc before I realized I forgot to put the fuel cut off valve into the on position.