[tuxiano]

Occasionally it is possible. After seven years and more than one hundred games played I can tell you that I have been surprised by my opponent's reply not more than an handful of times. For "surprised" I mean he didn't play the top choice of the engine. In fact most of the times the best move in a given position is easily agreed on by any reasonable engine on any decent hardware. In few critical moments in the game, the best move is not clear and there are two or three or more playable alternatives that take into very different positions. In these cases the computer, after a long thought (one or more hours) usually converges to one suggestion and sticks to it even if given more time (a sort of "horizon effect"). These are the moments where a human, after a long thought, can overcome the computer suggestion and favor the 2nd or 3rd choice of the engine. So in brief no, I can't recall a game where I've been gifted the win by my opponent "weakened" move while most of the time I have confronted with the "engine's approved" suggestion and had to build my win by refuting it.

[vecter]

I assume that when you come across one of these novel moves, plug it into the computer, and give it time to search, it ultimately decides that it's superior?

Relatedly, can you give some examples of novel non-engine lines that turned out to be better than engine lines?

[tuxiano]

Sometimes if you play a move and the first plies (i.e. half moves) of the main variation the computer starts "understanding" and its score changes accordingly. Those are the cases where more hardware power could be useful and make the engine realize the change from the starting position. More often, the "non-engine" move relies on some blindness of the engine, so the computer starts understanding its strength only when it's too late. In these cases is unlikely that more power could bring benefits. Typical cases are

- fortresses [0]. One side has more material but the position can't be won by the superior side. As the chess rules declare the draw only after 50 moves without captures or pawn pushes, current engines can't look this far away and continue manouvering without realizing the blocked nature of the position. Some engines have been programmed to solve this problem but their overall strength decreases significantly.

- Threefold repetitions [1]. The engine believes the position is equal and move the pieces in - let me say - pseudorandom way. Only at some point it realizes the repetition can be avoided favourably by one side. Also this topic is frequently discussed in the programming forums but no clearcut solution has still emerged.

If you are looking for positions where human play is still better than engine's, the opening phase is the most fruitful. Most theoretical lines were born by human creativity and I doubt a chess engine will ever be able to navigate the intricacies of the Poisoned Pawn Variation of the Sicilian Najdorf [2] or the Marshall Attack of the Ruy Lopez [3]. Neural networks engines are strategically stronger than classical AB programs in the opening phase but they suffers from occasional tactical blindness. Engine-engine competitions often use opening books to force the engines to play a prearranged variation to increase the variabililty and reduce the draw percentage.

[0] https://en.wikipedia.org/wiki/Fortress_(chess) [1] https://en.wikipedia.org/wiki/Threefold_repetition [2] https://en.wikipedia.org/wiki/Poisoned_Pawn_Variation [3] https://en.wikipedia.org/wiki/Ruy_Lopez#Marshall_Attack