I would add as my favorite inexcusable blunder the failure of the axis (both Germans and Japanese were totally guilty) in protecting codes. The arrogance/ignorance/carelessness of the Germans in believing that the allies were incapable of cracking their codes has always been shocking to me. May have been a different war if the axis would not have laid out sub locations/fleet strategies for the allies. The tide of the Battle of the Atlantic would certainly not have changed for several more months and who knows what may have happened at Midway or for that matter Coral Sea.
That's a good one that hasn't come up so far. The Germans did an average job (at best) with signals security. Naval codes were compromised almost on a routine basis. But that happened to all the players in the war. Just more widespread and more players working at decoding German signals.
Pretty interesting idea.
It is also interesting that German Naval Codes were compromised in both World Wars and, in both cases, the Naval High Command refused to believe their messages were being read by their Enemies. Even after review by intelligence agencies.