Blast from the past. Broken Arrow Accidents...

via AtomicHeritage.org

At 11:50 a.m. local time on May 22, 1957, a B-36 aircraft jettisoned an unarmed Mark 17 ten-megaton hydrogen bomb over Albuquerque, New Mexico. The thermonuclear device, weighing 42,000 pounds, was being transported from Biggs Airfield in Texas to Kirtland Air Force Base just miles south of Albuquerque.

Accounts of what caused the incident vary, but one version suggests that a crewmember in the bomb bay was jolted by sudden turbulence. He grabbed hold of the manual bomb release lever to steady himself, causing the weapon to fall through the closed bomb bay doors and plummet to earth.A Mark 17 bomb, the type accidently dropped at Kirtland

The nuclear chain reaction necessary to set off the bomb did not occur because the bomb’s fissionable plutonium component was stored separately aboard the aircraft. However, the device’s conventional explosives detonated, leaving a crater 12 feet deep and 25 feet wide on an uninhabited area of land owned by the University of New Mexico. The Field Command division of the Armed Forces Special Weapons Project, responsible for recovery and cleanup operations, reported the incident’s only casualty was a nearby grazing cow, and found that radioactive material did not spread beyond one mile of the crater.

The incident was revealed to the public for the first time in the 1980s, after the Air Force released declassified documents under the Freedom of Information Act (FOIA). AHF Board member Robert S. Norris, then a research associate for the National Resources Defense Council, remarked that the Mark 17 “is possibly the most powerful bomb we ever made.”

More here. 

Close calls.  Many close calls that we know of and probably even more that we'll never hear about.

Add this to what goes on in the other nuclear powers and its amazing that we haven't seen a catastrophic event.