A “Broken Arrow” is defined as an unexpected event involving nuclear weapons that results in the accidental launching, firing, detonating, theft or loss of the weapon.
Six cruise missiles armed with nuclear warheads were loaded onto a US B-52 Bomber in North Dakota. The protocol prescribed to check that the cruise missiles were not armed was neglected at multiple instances resulting in a thirty-six hours period, during which no one in the Air Force realized that six nuclear weapons were missing.
Further materials: