user20224: Are there examples of chess games where the exact losing move has been found only with the help of engines?
Better still, games where the exact losing move cannot be found even by the best current engines?

user15223: Sure, examples exist. E.g. Game 6 of Carlsen-Caruana, World Chess Championship 2018:
[FEN "5k2/8/5pK1/3B1P1P/3n4/8/3b4/8 w - - 6 67"]


Kg6? loses, but no human would have found the refutation.

As for games where the exact losing move cannot be found even by the best current engines - I spoke to a correspondence chess expert recently, and I'm confident they don't exist. Given a decisive game, an expert correspondence chess player will always be able to find the losing move, but it will not be trivial, and a lot of analysis is required.

20224: <p>Are there examples of chess games where the exact losing move has been found only with the help of engines?</p>

<p>Better still, games where the exact losing move cannot be found even by the best current engines?</p>
 15223: <p>Sure, examples exist. E.g. <a href="https://en.wikipedia.org/wiki/World_Chess_Championship_2018#Game_6:_Carlsen%E2%80%93Caruana,_%C2%BD%E2%80%93%C2%BD" rel="noreferrer">Game 6 of Carlsen-Caruana</a>, World Chess Championship 2018:</p>

<pre><code>[FEN "5k2/8/5pK1/3B1P1P/3n4/8/3b4/8 w - - 6 67"]
</code></pre>

<ol start="67">
<li>Kg6? loses, but no human would have found the refutation.</li>
</ol>

<p>As for games where the exact losing move cannot be found even by the best current engines - I spoke to a correspondence chess expert recently, and I'm confident they don't exist. Given a decisive game, an expert correspondence chess player will always be able to find the losing move, but it will not be trivial, and a lot of analysis is required.</p>
