http://en.wikipedia.org/wiki/1966_Palomares_B-52_crash
Mid-air refueling accident. Four bombs. Three came down on land. One went in the drink and was recovered 2½ months later.
Two of the three that hit land exploded, although there was no nuclear yield, just a big radioactive mess.
The reason why there was no nuclear yield is an interesting topic in itself. Nuclear weapons are designed to be "one-point safe". That is to say that, if the shaped charges around the plutonium core are not all detonated at the same time, the core (and the rest of the bomb) will be destroyed, but no nuclear explosion will happen. See the Wikipedia on Warhead design safety.
