Failure to take Moscow during the initial pushes. This was (in my opinion) the lynch-pin.
First defeat and all. Showed the Germans that they were not super-human, and the Russians that they could stop them/win.
I don't believe there was any stopping Hitler and his quest for Liebesraum, so the start may not be a real option, or I'd have picked it and gone farther and said the start of the whole shooting aspect.
Moral is more important than most consider it to be. It's kinda like "trust", difficult to obtain, simple to lose, and almost impossible to get back.
If Moscow fell, the domino theory may have taken place in Leningrad/Stalingrad.
Connect the dots between the three, and that's all Hitler wanted. Well, not all, but enough for his original ambitions.
Add Stalingrad and it's way over.
Kursk was adding insult to injury.
Foolish pride...perhaps stubborn and vendictive as well.

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There's one way to find out if a man is honest-ask him. If he says "Yes", you know he's a crook. Groucho Marx