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Some interesting replies in this thread.

Here are the biggest mistakes as I see them.

Axis; Not supplying Rommel with the extra panzer division that he desperately needed to take Egypt in early 1942. If Rommel had taken Egypt the Germans would have gained complete control of the Mediteranean, (no more English naval bases, apart from Malta which would have been quickly dealt with), but more importantly they would have gotten control of the Middle East oil fields. One of the major reasons that Germany lost the war was a lack of oil. The German air force was effectively disbanded in 1944 as they had no more aviation fuel.

The second Axis mistake can be blamed on good old Mussolini. By invading Greece in early 1941 and getting in a lot of bother, he forced Hitler to come to his help. Hitler had to send divisions which were preparing for the attack on the Soviet Union. This delayed the invasion of Russia by six weeks. Which was just the amount of time they needed to get into Moscow.

One Allied mistake has already been mentioned - Yalta. The second major cock-up was Singapore. This British naval base was the key to South-East Asia. The attacking Japanese army was outnumbered and having a hard time when the British commander lost his nerve and surrendered, thus handing control of the Dutch East Indies with their rubber plantations to Japan. If Singapore had held, the Pacific War would have been greatly reduced.