A mysterious fuel leak, which the crew found out about as a refueling plane approached, led to the broken arrow incident over North Carolina in 1961.
The leak quickly worsened, and the jet bomber "lost its tail, spun out of control, and, perhaps most important, lost control of its bomb bay doors before it lost two megaton nuclear bombs," according to a two-part series about the accident by The Orange County Register. "The plane crashed nose-first into a tobacco field a few paces away from Big Daddy Road just outside Goldsboro, N.C., about 60 miles east of Raleigh."
One bomb safely parachuted to the ground and snagged on a tree. Crews quickly found it, inspected it, and moved it onto a truck.
However, the parachute of the other bomb failed, causing it to slam into a swampy, muddy field and break into pieces. It took crews about a week of digging to find the crumpled bomb and most of its parts.
The military studied the bombs and learned that six of seven steps to blow up one of them had engaged, according to The Register. Only one trigger stopped a blast – that switch was set to "ARM" yet somehow failed to detonate the bomb.
It was only "by the slightest margin of chance, literally the failure of two wires to cross, a nuclear explosion was averted," a declassified 1963 memo described Robert McNamara, the secretary of defense at the time, as saying.
"Had the device detonated, lethal fallout could have been deposited over Washington, Baltimore, Philadelphia, and as far north as New York City – putting millions of lives at risk," according to a 2013 story by Ed Pilkington in The Guardian.
Here's a Nukemap simulation of what the blast radius and fallout zone of the Goldsboro incident might have been: