The only "shock" that I've ever felt was from a muscle stimulator. My friend got into a bad accident a few years ago and he has a few metal pieces and parts in his back. Overall, his back is pretty fucked up. Anyway, he has a muscle stimulator to help "massage" and relax his back muscles when they tighten up on him.
So, he brought it to school one day and was letting people play with it, sort of. It was just this box that had a dial on it that went from 1-10 and it had two nodes on it that were at the end of little wires, which were stuck to your skin. Anyway, he put it on my hand, just to show me how it felt. He turned the dial to 1 and I was like "OK, this is alright". Then, he turned it up to 2 and I was like, "OK, this feels kind of good". Then, he decided to be a fucking ASSHOLE and he turned it all the way up to 10. I swear to God, my muscles contracted so hard that it felt like my hand was breaking. I instantly dropped to the ground and was like "AAAAAAAAAAH, you FUCKER!!!"