I got zapped once as a kid. My grandfather's TV, I was pulling the tubes to take them to the testing machine. Fairly small screen but I knew it probably had some residual charge that I most definitely didn't want to come into contact with. The layout was such that I could pull the tubes while staying far away from it. It never occurred to me that safety would require keeping him from looking on in curiosity and bumping my arm. (He was impatient about getting his TV fixed, I saw that whoever designed it had had the courtesy to put the tubes in an area where even if your hand slipped it wouldn't approach the tube. That was no protection against my hand getting moved in a direction I was not applying force, though.)

I don't think such things are actually capable of direct lasting damage but you do lose control--I slammed my hand into the cabinet pretty hard, completely involuntary. My understanding is that tasers are AC, thus involuntary movements will be back and forth rather than just in one direction.