My first was when I was around 4 years old and in Germany and I stuck a random piece (paper clip or hair pin) of wire into a receptacle in the hallway, getting jolted in the process with 220 volts, of course.
My then-nine year old cousin asked me what happened and between sobs I pointed at the socket on the wall with the little paper clip sticking out of it. Fool goes to grab it and pull it out and soon starts...feeling my pain.
The most recent one was two summers ago when I was replacing worn-out/cracked wall sockets at my mom's apartment.
I had gotten to the point of replacing the dedicated 20-amp/110-volt air conditioner socket.
I didn't test for voltage before prying off the painted-on wallplate (which came off with the receptacle face ADHERED to it from al the paint).
Didn't realize I had pulled the wrong 20-amp breaker (incorrectly labelled too) before sticking my slightly sweaty hand in the surface-mount raceway box and grabbing for the remainders of that damned socket with such gusto!!!
I let out a loud curse and my arm vibrated for a bit after I yanked it out. My grandmother (in the kitchen at the time) was like "so, it bit you?" "Uh-huh," sez I.
Sho-nuff...when I went over to the breaker box, I found out I had pulled the adjacent 20-amp breaker that powered a dedicated Nema 5-20 outlet in the kitchen instead!!!
For that project I had been replacing outlets branch by circuit and testing each one individually after it was installed and powered on, instead of knocking the whole house out while I worked and thereby cutting power off to an icebox full of perishables.
Always use the voltage tester (TWICE) before sticking your hand in the damned box!!!!
It's the stupid things like that that can kill you...