That kind of accidents are only possible for gun-type bombs. For an implosion-type bomb to initiate, the explosives really do need to all go off simultaneously, detonation from one side just causes the core to be sprayed out through the other side. Implosion bombs are designed to have the least possible amount of fissiles for weight/economics reasons, which means that even a sub-millisecond delay in the detonation of one of the slow explosives causes the core to completely fail to initiate.