As the old saying goes, "It's never over till it's over !" Here is the perfect example:

In this position

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White is completely winning in this chess game! All he had to do was play 68. Ke6 and black cannot avoid both threats of Rh8 mate and e8 (Q)!

But white made a horrific blunder and this chess game could have been over immediately! White decided to be cute and hallucinated! He played 68. Rh8+??

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Now, all Black had to do is play the ONLY legal move 68...Kxh8 and would be completely winning in this chess game! But for whatever reason, Black also hallucinated thinking that he was losing this chess game! So, he resigned in a completely won chess game!

So what is the moral of the story? It is never over until the chess game is truly over! It can happen even among two chess grandmasters!