Could you clarify what you mean by this? There is a common trope around 'not invading Russia in the winter' or 'being defeated by the Russian winter' which you might be thinking of; however, this is not really a satisfying explanation for the Nazi defeat in the Soviet Union. The harsh winter conditions certainly did play a part in stopping some Wehrmacht advances, and it was very detrimental to their mobility, morale, and combat effectiveness. However, the effects of winter on the German armies were symptomatic of their broader lack of realistic preparation for the invasion, rather than being a root cause of the defeat. The tenacity and skill of the Soviet military must also be given its due. Yes, the Soviet leadership made extremely serious mistakes during the initial period of the war, and the Red Army of 1941 was in quite poor shape in terms of equipment, organization, manpower, and morale. In spite of this, they reformed themselves quite well and by the end of the war (even by 1943, arguably) were a battle-hardened, well-equipped, highly motivated and professionally competent military. This competence was reflected in, for example, the increasingly complex and skillful maneuvers imposed on the Wehrmacht by the Red Army, most famously the 1944 Operation Bagration offensive which annihilated Army Group Centre and swept Nazi forces clean out of Belarus & parts of Poland.

I'm quite curious what mistake you're referring to though; I can give a much deeper answer if you can clarify.