I had my first and only ND (negligent discharge) a year ago. Nobody and nothing was hurt, except my pride and a roof shingle over my bedroom.
I had been installing some new rosewood grips on a Colt Officer 1911 that I had inherited from my dad. The original one with the Colt emblem crumbled, and my wife bought me the new grips for my birthday. After I installed them, I took my Glock out of my holster and laid it on the bed. I pulled back the slide of the Colt, looked through the port and saw that there was no round and no magazine, then holstered it and practived a dry-fire. Satisfied, I put the Colt in its box and locked it.
I noticed my Glock laying on the bed. As if it was a habit, I picked it up, racked the slide, pointed at the ceiling and squeezed the trigger. (To this day I have no idea why I would ever do that.)
The two loudest sounds a gun can make are: 1) Click when it should go Boom, and 2) Boom when it should go Click.
Of course I was deaf. I was also stunned, as if, "What? How the heck could that have happened?" It took about 2 minutes before I realized my wife, in the living room, would be wondering 1) if I was alive, 2) what did it hit? and 3) if she came in to check on me, where was it now pointed? (Good on her - I wouldn't have thought that one.) I shouted "I'm okay!", put down all weapons, and went in to try to explain. Turns out she had been shouting my name, and "Are you okay?", which I did not hear.
I locked up all my guns and didn't touch one for a month. My ND changed forever my attitude towards handling. The Four Laws which I had memorized, had become VERY REAL.
1) Treat ALL guns as though they are ALWAYS loaded.
2) Don't allow the muzzle to point at or sweep anything that you are not willing to destroy.
3) Keep your finger off the trigger until have the target in your sights and are ready to destroy it.
4) Know your target, and what is behind it (and in front of it, and beside it).
Because I am such a poor shot, I have added a corollary to #4: When you miss, you will still hit SOMETHING. You are responsible for whatever that is that you hit.