In 1961, a B-52 broke up over North Carolina releasing two nuclear bombs. A single switch prevented detonation
1961 was during the height of the Cold War. During this time, the United States was trying its hardest to keep U.S. nuclear arms airborne and deliverable 24-hours a day.
A B-52 bomber which was out on one of these "airborne alert" missions over North Carolina actually broke up in midair while carrying two nuclear weapons. One of their parachutes deployed, but the other fell the 2,000-10,000 feet in freefall and broke upon impact. Five of the eight crewmembers survived.
The official statement said that these bombs were "unarmed," but that's misleading. There are several steps in arming a nuclear weapon, and both had been partially armed. The one where the parachute failed actually came one step from being armed due to the fall, according to some accounts.
Nuclear physicist Ralph Lapp said that "five of the six interlocks had been set off by the fall…" and thus, "only a single switch prevented the bomb from detonating and spreading fire and destruction over a wide area."
Some argue against this, but there are multiple testimonies agreeing with Lapp. Someone working at the Pentagon at the time said he saw a classified document that confirmed this. He said that when the behavior of safety features in both bombs involved in this incident are taken into account, every kind of safety interlock had failed.
Additionally, a former Secretary of Defense stated that "The bombs' arming mechanism had six or seven steps to go through to detonate, and it went through all but one, we discovered later."