I'll call a recent incident I had my first ND. At the range, my wife was having difficulty with my Colt Model 22. She just couldn't group her shoots worth a flip. I went to her lane and picked the gun up, visually inspected the sights (which looked fine) sighted in on her target, and fired. The bullet went just where it was supposed to go - down-range through the target and into the backstop.
Why was this an ND? Because I didn't notice that my wife had taken her ear muffs off when I came over to her lane!
That happened over a week ago, and I am still beating myself up about it. Thank God it was a .22LR, and not her 9mm or my .40SW!