We had one of those WOW moments the other evening, when it was discovered that a US B-52 bomber had accidentally dropped two hydrogen bombs while breaking up in the air. Both armed themselves and one got within one low voltage switch of detonating. Obviously, both didn't, and fell to Earth relatively harmlessly.
In 1983, I remember being between jobs, and having a conversation where I was wondering whether it was worth looking for another, as the nuclear cold war situation had got to freezing point. The defence condition of the opposing powers had gone up a notch or two. Today I read this, and it doesn't surprise me. I think it happens more often than we think, but not under those conditions:
http://www.bbc.co.uk/news/world-europe-24280831