I've never had a ND yet.
Like others here, I have had an "Oh Sh*t" moment when I realized I almost screwed up. Like the time I was shooting a friend's revolver, trooper special or some such thing, loaded it, cocked the hammer and then swept my friend's legs. When I raised the pistol to fire, the trigger was amazingly light. I realized how easy, with that trigger, it could have been to blow a hole in my buddy's leg.
I did witness an ND that scared the crap out of me. Was at a friend's ranch where there were 6 guys hunting. My friend and I were visiting and all 6 hunters loaded into a 4-door pickup about 40 yards away. All the doors close, and a moment later, kabooooom! Like fricken artillery went off and the rear window just disintegrated. We looked at each other and thought, no way we are going up to that pickup. About that time, all 4 doors are thrown open and the 6 guys bailed out.
Come to find out, a 16 year old in the back had a .300 mag, butt down, barrel up in the back middle seat. Leaned the barrel back between him and guy to his side. Somehow he torched it off. Luckily, it went off between the two and right out the back window.
The kid's dad could not comprehend why my buddy told him he and his hunting buddies had to leave that afternoon.
My friend explained that where we lived and what we do, firearms are tools used everyday. With our families around us constantly, we do not abide by people who make mistakes with firearms. Too much at stake. I'm sure that guy still doesn't get it.