I had 1 ND when I was about 12. Growing up my dad was very strict about muzzle dicipline but I wasn't ever exposed to all the safety precautions I take now. One day while at the house by myself I was messing around with a 35 Remington pump action that I deer hunted with. Typically our guns were never loaded in the house but for some reason, on that particular day, the 35 was. When I picked it up and saw that it was loaded I proceeded to unloaded it in the house (which I was taught NOT to do). The only way to empty that gun is to cycle the rounds out one by one so I pointed it towards the ceiling, ejected the first round and sent the second into the chamber, apparently with my finger on the trigger. After the deafening report from the muzzle and the famous WTF moment, I saw a ray of light coming from outside and realized just how close I had come to seriously jacking up my world. It took several minutes of standing there like a statue before could muster up the courage to put the safety on, walk outside, and clear the gun safely. That was possibly the scariest moment of my life. Knock on wood, that is the only one I have had.
I witnessed one at a class a couple years ago that shook everyone present to the core. Throughout the class I was on the line next to a guy who could not, for the life of him, keep his trigger finger out of the trigger guard of his Sig when it didn't need to be there. It got bad enough that towards the end of the class the instructor warned him that if he didn't get his problem fixed asap he would have to leave and wouldn't be allowed to come back. That statement actually worked for the remainder of the class until the end of the class when we had a competition to put into use what we had learned that day. The competition included shooting and moving from different positions, several mag changes, and ended with 2 shots on a swiveling hostage target from behind cover. The first shot on the hostage target was to be fired around the right side of cover, then a transition to weak hand, and another shot on the target from the other side of cover. Apparently, everything this moron ever learned about safely handling a handgun went out the window halfway through his run, under the crushing stress of a shot timer as he was moving across the range to engage the hostage target. As he went to transition to his weak hand he apparently left his finger inside the trigger guard, again, which caused him to put a round just between his feet when he aquired his weak hand grip. Due to the narrow stance he had taken as not to expose his feet around the cover, the 9mm round was only a couple inches from either foot. To further add to the horror of the situation, myself and several other students observed this collosal F up from the deck where we had all landed after seeing the business end of this guy's Sig as he was running across the range to the last barricade. I hate to imagine what could have happened had that ND happened while his muzzle was pointed in the direction of the rest of the class.
To the instructors credit, he tried everything in the world to safely make the guy better. I understand in his mind that is his job but I wouldn't have minded to finish the class without that individual. He also couldn't see the guy muzzle us from where he was (following with the timer) before he almost shot himself in the foot. After the class I told the instructor that I planned on taking another one of his classes as long as that guy wasn't there to which he promptly informed me that he would not be back.