I don't want to get into a semantic argument of AD vs. ND so I'm just going to use the term AD...
Years ago, I heard my shooting instructor say, "There are two types of shooters, those who have had AD's and those who will have AD's... May all your AD's be down range."
From what I've seen, he's right. And I've seen some very well trained people have a round get away from them on occasion. Sooner or later you are going to have a moment where a round is fired before you were ready. It will end up in a spot that is not where you wanted to place it. It's not a horrible thing if it's in a safe direction. It will shake you up. -One of the reasons you train to keep your muzzle pointed in a safe direction is for the day when you do have an AD.
Regarding AD's from mechanical failures, it's not that uncommon. All it takes is a gun with a broken part, a missing part, or a "Bubba'd" one from "Adventures in Home Gunsmithing" and you can have a firearm that will AD.