It happened to me. I read once on the forums at the now-defunct thefiringline.com that if you've never had an AD or a ND, it's because you really haven't owned a gun that long.
I was over at the compost heap, hoping to score a shot on a pest or two. Had a chance to shoot an arctic hare (their hair is starting to turn brown for the season), but since I don't believe in the wanton destruction of animal life (apart from food or pest control), I passed it up.
Since no pests were hanging around the heap, nor the pig yard, I decided it was at least time to get my monthly shooting in, at least to keep my accuracy up. My last shooting session was just to test out handloads, so I wasn't all that interested in really shooting for accuracy.
This time, it was focused tin torture. A medium sized soup can at 15 paces. I was hitting it every time (God, I love my H&K).
I emptied one magazine, then loaded another. Hit the slide release, and like a dummy, got into fondling the trigger (with a loaded chamber, with the safety off, and the hammer cocked). I was turning my head to see where the boys were behind me, and "blam!"--the earth eats 230gr FMJ about a yard in front of me.
I secure the weapon, and sheepishly tell the boys that this is the reason why you keep your finger out of the trigger guard until you're ready to fire.
Happily, no one, including me, was hurt, but it was a sobering reminder regardless of your experience level, you always need to keep your safety awareness up.
Then I got back to perforating tin. Again, hit it every time. One of the kids challenged me with a smaller target--shoot the top of the can instead of the sides.
I obliged him. Blam! The can jerks upright from its side, and stands there. After securing the weapon (decock, safety on, pointed at the ground), we inspect the shot.
God, I love my H&K...