Search Google for information about taser incidents. They are mostly used by police, and most of the time, officers seem to use them correctly, which means ONCE. That's how they are tested and thus rated "non-lethal". But it's not uncommon for a sadist cop to find an irresistible chance to taser someone a half-dozen times. Seeing someone writhing helpless on the ground just seems to trigger them to keep shooting. This often results in serious injuries even if it's not fatal. Comparable to people injuring themselves during epileptic seizures. But more importantly, it is very possible to kill someone by reflexively tasering them over and over. Just like a defibrillator can stop your heart as easily as restart it, so can a taser.
Most civilians have even less training than police, of course. Meaning they are even more likely to lack restraint, resulting in the same behavior as the sadist cops - just a monkey with a device that makes the other monkey twitch while they gawk.