All kinds of blunders in WW2, but I think Hitler switching the Luftwaffe to target London takes the cake. He could have taken out England and invaded Russia resting secure in his left flank. Not retreating from Stalingrad was a pretty devastating one, too.
On the flip side, not waking up Hitler on D-Day was a mistake.
Operation Market-Garden was a mistake, and frankly it angers me. The entire operation is a result of Montgomery being a <insert your favorite less-than-positive word here> and the commanders making a bet whether the Americans or the British would reach Germany first. Good men died over a bet.
The Sherman and the M-10 tanks were a big mistake. What idiots sat comfortably in the Pentagon designing this stuff?
One could argue that Pointe du Hoc was a big mistake, but I don't attribute that one to idiocy. That was just good counterintelligence on the Germans' part, just as the OSS misled the Germans into believing the invasion would happen at Pas de Calais.
Invading Normandy at all I say was a big mistake, and probably politically motivated. Invade Norway or south France first--they could have landed virtually unopposed. The Germans have to move troops and armor away from the north France beaches to counter the threat. And guess what happens when they move anyone, anywhere? P-47's swoop down and take a bite out.
Not exploiting the Anzio surprise landing was a big mistake, but anyone could have made it. MacArthur learned the lesson from it and didn't repeat it at Inchon.
I call all of Russia just one big mistake. Hard to cite mistakes by Stalin, since I never had high expectations of Stalin's leadership to begin with.