A recently declassified Sandia National Lab report published by the National Security Archive offers new details about a 1961 nuclear weapons accident in Goldsboro, North Carolina.
While both multi-megaton Mk 39 bombs involved in the mishap were in the "safe" position, the report concluded, by the time one of them hit the ground it was in the "armed" setting because of the impact of the crash. If the shock had not also damaged the switch contacts, the weapon could have detonated. Since the advent of the nuclear age, the nightmarish possibility of an accidental detonation has made weapons safety a boiler-plate item in the U.S. nuclear weapons program — yet potentially serious errors continue to occur. A series of 2013 reports on the Goldsboro accident provided a fresh reminder of the role of luck in preventing nuclear disaster: the same switch involved in the 1961 event had failed in other incidents.
More at the National Security Archives website.