Voted for the last option. The Stalin government's lackluster preparedness for Barbarossa cost the USSR not only 20+ million lives, but also the chance for even greater presence in Europe and a stronger economy (due to more of their pre-war industrial capacity remaining intact) following the war. Not exactly the worst mistake for the Western Allies and their future capitalist cold warriors, but a mistake nonetheless.
Since this poll is Eurocentric, I'll mention some other mistakes from the Asia-Pacific theater:
-The KMT's lack of preparedness for war with Japan - which I guess ties into underestimating japanese military strength. Admittedly, this is due to the piss-poor state of internal affairs both within the KMT and across China, what with all the corruption and civil war against the communists. Still, as ASB as it may seem, a longer 1920s KMT-CCP peace would have left the country in a better position to repel the Japanese Empire - a case of hindsight being 20/20.
-Douglas MacArthur.
-The decision to divide the Korean peninsula into Soviet and American occupation zones, thus setting the conditions for the Korean War. If any country in the East deserved to be divided post-war, it was Japan - not Korea, and certainly not Vietnam later on either.
-The incomplete punishment of Japanese war criminals and Japan's wartime political/economic establishment by the US occupation. Today's Japan would have better relations with Korea and China if their political climate didn't have room for people like Shintaro Ishihara, Toru Hashimoto, and Shinzo Abe (all of whom are nationalistic right-wingers who deny/downplay/do apologetics for their country's WWII crimes).