io9 reports on two bombs that were accidentally dropped on the Carolinas in 1958. (They didn’t explode.)
io9 on 5 other times “we almost nuked ourselves. . . . The sad lesson is that we have less to fear from naked aggression than we do from incompetence and bad engineering.”
And a fairly authoritative and comprehensive account of U.S. Nuclear Weapons Accidents.
The Hyperarchival Parallax 