Oddly, the US Department of Energy's statutory responsibility for "the safety and reliability of the nuclear arsenal" is officially interpreted within the department as: safety, it goes bang when you push the button; and reliability, it makes as big a bang as it says on the tin.
Of course this "safety" has absolutely nothing to do with what the public naturally assumes is meant, which is typically that it won't go bang when you don't push the button, or leak, or be hijacked, or fall out of a plane carrying it. The DoE considers none of those its responsibility under the law.
One might guess that going bang when the button is not pressed would be extremely unlikely. But when a B-52 crew made a booboo and dropped a pair of hydrogen bombs in a farmer's field, investigation showed that all of the processes that would have set them off completed; just not all on the same bomb.