There was also the time the United States Air Force accidentally nuked South Carolina.
Fortunately, the fission core was out at the time, otherwise it would have gone off.
I see your South Carolina bomb, and raise you one North Carolina bomb:
http://en.wikipedia.org/wiki/1961_Goldsboro_B-52_crashThe two nuclear weapons separated from the gyrating aircraft as it broke up between 10,000 and 2,000 feet (3,000 and 610 m). Five of the six arming mechanisms on one of the bombs activated, causing it to execute many of the steps needed to arm itself, such as charging the firing capacitors and, critically, deployment of a 100-foot-diameter (30 m) retard parachute. The parachute allowed that bomb to hit the ground with little damage.
According to former military analyst Daniel Ellsberg, he saw highly classified documents indicating that the pilot’s safe/arm switch was the only one of the six arming devices on the bomb that prevented detonation.[1][5] The Pentagon claims that there was no chance of an explosion and that two arming mechanisms had not activated. A United States Department of Defense spokesperson told United Press International reporter Donald May that the bomb was unarmed and could not explode.[5] Later, however, it was found that both bombs were fully functional.
Yeah; 2 3.8 megaton active thermonuclear weapons, one of which had 5 of 6 arming mechanisms activated in the crash. North Carolina nearly turned into a smoking hellcrater.
The real thing to keep in mind with such nuclear weapons incidents is they almost certainly won't set off a nuclear explosion, even if the triggering HE explosives go off. If an impact detonates the HE explosives, it isn't all at once, the shock from impact and explosion of other HE bits takes some time to travel through the device, making for an asymmetric detonation. This will largely throw the nuclear material around, but it won't result in the perfectly timed, entirely symmetric implosive detonation required for the fission to begin. Which is why, out of all the nuclear accidents, that north carolina one is IMO the scariest. If that sixth arming mechanism had jiggled just a bit, similar to how the other 5 had, BOOM. A properly trigger explosion which would actually result in a full thermonuclear detonation. Even if the core had been in that South Carolina warhead, it would only result in a dirty bomb-like explosion, rather than a nuclear detonation.