I believe that when the Germans were nearing Moscow the Russians sent out peace feelers to Hitler who turned them down. If this is true, then as serious mistakes go, that was as big as they get.
There was an interview on our TV about 8-9 months ago and one of the people they interviewed was Stalins translator. He didn't talk to the Germans it was one of his trusted aides, who was shot for treason shortly after. Th implication was that dead men don't tell tales.