I have never had a ND, but I was present when one happened with my pistol. Back in 1981, I had a brand-new PI license and permit to go with it, and a brand-new Chief Special Airweight. I was visiting a friend who was a brand-new cop, and I had to show him my pistol. I removed the rounds in front of him and set them on the table, then handed him the gun with the cylinder open. I then turned around to watch the cartoon that was on the TV (it was Saturday morning, c'mon). We chatted a bit, but I wasn't watching what he was doing and really was paying more attention to the cartoon. I heard him say "how's the trigger pull on this?" BOOM. Less than 6 inches from my ear.
After the freaking out and cleaning up (it was his parents' house) the sheetrock dust and insulation that had come from the ceiling, I was able to ascertain that as we chatted and I watched cartoons, he had a brain fart. He said that while we talked, he put the rounds in the gun, snapped the cylinder shut, and pointed it at the big picture window above the TV. Then for some unknown reason, he actually forgot that he had just loaded the weapon. This is the "how's the trigger pull" point, where he squeezed one off. Not expecting it to fire, he was holding it loosely and it bucked upwards, putting a one-foot groove in the ceiling before the bullet got to the insulation and finally the roof joist and stopped.
How do you guard against a complete mental lapse? If he could forget that he had JUST LOADED THE DAMN THING, what would stop him (or me, or anyone else) from forgetting any other rule? I can, however, attest to the fact that safe handling got real for me that day. Saying "I cannot possibly have a ND" is the same as saying "I cannot make a mistake." Period. I happen to make them. All of my life. I hope that by being very careful and trying very hard to follow every rule I will minimize the chance of hurting or killing someone (including me) if and when I do make one mistake, and that the other rules I didn't make a mistake on save my butt. I don't expect to have a ND. Ever. But I could.