I have never had a ND. I know people who have. No one was injured, however, because they were observing other safety rules. For instance, I participate in cowboy action shooting. I've seen a couple of ND's there, including my own father who let one go at the loading table. His thumb slipped while trying to lower the hammer, and unfortunately he had indexed the cylinder wrong and a live round was under it. He put a .45 caliber hole in the loading table, and scared the bejeezus out of the loading table officer. No one was hurt, however, because he was observing rules 1 and 2, i.e., treating all guns as if they were loaded (he thought there was no live round under the hammer, but was careful, anyway) and keeping the muzzle pointed in a safe direction. Thus, no one was hurt.
ND's can indeed happen. There's no excuse for killing someone with an ND, however, because safe gun handling means not exposing anyone else to the risk of getting shot by your own carelessness.