My understanding is that all of the safeguards worked as intended and characterizing them as 'failed' is a bit unfair. The three that "failed" were designed to

My understanding is that all of the safeguards worked as intended and characterizing them as 'failed' is a bit unfair. The three that "failed" were designed to prevent the bomb exploding on the runway (before take-off), on the plane (before being dropped), or in the air (before the plane could escape). Since this event involved a plane taking off, dropping the bomb, and the bomb parachuting to the ground - it was largely indistinguishable (to the bomb) from an actual intentional event. Only the cockpit Safe/Air/Ground selector prevented detonation. Note that the cockpit selector did fail in other incidents, arming bombs while still in the 'safe' position due to a short circuit elsewhere.