Before I came up with my answer, I first had to define "blunder" with regard to WW2 events. It seems to me that if you have a decent plan, get all the available intelligence, have the appropriate forces to commit, and coordinate everything correctly, you should have a decent chance of success. That being said, a case could be made for the attack on Pearl Harbor, the German declaration of war, and just about any other one on the list. Except one. As Thorlifter said, Operation Market Garden was a cluster F from beginning to end. First of all, Montgomery was an idiot. I have no idea how he gained the reputation that he has enjoyed. He never won a single decisive victory except when he had overwhelming superiority in men, material, and terrain, and even then, it took him forever to act on his advantages. Market
Garden was amateurish in concept. The idea that all those bridges could be seized and that XXX Corps could waltz single file down a single elevated road (like ducks in an arcade game) with no trouble should have been laughed at. When the intelligence came back showing tanks and SS in the area it was ignored. When the radio equipment looked like it might not work, that was ignored. And if these things weren't bad enough, after the whole thing got off to a disasterous start, they still kept dropping troops into the meat grinder. I can't think of a single thing worthy of praise except for the utterly heroic efforts of truly brave men who tried to make something good out of what was really a pile of dog crap. After all was said and done, Monty was knighted. Huh?