Thankfully, I haven't had any NDs since I've been an adult, and sure hope I don't; though I know it's likely and religiously follow the 4 rules. When I was about 10 or 11 though, I did. I was intensely interested in how my Dad's lever action Marlin .22 fed a round from the tube magazine into the chamber. So in order to observe it, I covertly obtained a round, and began dropping it down the tube, and loading it into the chamber; and subsequently ejecting it.
After a few permutations of this, it was chambered and somehow I accidentally pulled or snagged the trigger. It shot into the floor, thankfully no one was downstairs at the time, but subsequent probing showed it failed to penetrate the floor, stopped by a joist perhaps.
My parents thought I had popped a balloon; but I fudged on the truth a little and said I didn't know it was loaded. I received a lecture on the 4 rules, checking the chamber, etc.. But I was scared enough by the incident to be much more responsible with guns from that time on.