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What is the most obvious move, (or simple sequence of moves) long overlooked by opening theory in the last 50 years or so, completely changing the evaluation of a well-known position?

EDIT: Well, I mean those cases in which "an unknown amateur from Siberia wrote to the famous chess magazine that White is losing, because after the simple 12...Nxc3, White cannot take back, because of the discovered check..." Are those just urban legends, or they did actually happen? I am curious...

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    This question is very broad and will probably be closed if it isn't edited. – Aric Aug 17 '17 at 13:04
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    There are too many to list. Just listing the computer's breakthroughs would take pages. – Fred Knight Aug 17 '17 at 13:29
  • We have had GMs of incredible strength for 100+ years. There are no simple overlooked moves in the opening phases of the game. – Tony Ennis Aug 17 '17 at 23:14
  • @TonyEnnis No doubt there are no simple overlooked moves in the main line openings that those incredibly strong GMs have been playing for the past 100 years, but maybe in some obscure variation of the Steinitz Gambit or the Anderssen Counterattack? – bof Aug 18 '17 at 2:27
  • There is a game presented in Starting Out: the King's indian Defence where a line was considered lost for Black, but there is a simple move that results in a draw. I'll dig up the example when I'm back home. – user1108 Aug 18 '17 at 9:17
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Joe Gallagher says of 22... Kf8 in Starting out: the king's indian:

Vaisser relates how for many years this move was thought to lose because of some erroneous analysis.

Of 25...Qf8 he writes:

This came as a great shock for Vaisser. According to theory at the time Black was supposed to play 25...Ra6 and lose

[FEN ""]
[White "Vaisser"]
[Black "Bauer"]
[Startply "50"]

1. d4 Nf6 2. c4 g6 3. Nc3 Bg7 4. e4 d6 5. f4 O-O 6. Nf3 c5 7. d5 e6 8. Be2 exd5 9. cxd5 Re8 10. e5 dxe5 11. fxe5 Ng4 12. Bg5 Qb6 13. O-O Nxe5 14. Nxe5 Bxe5 15. Bc4 Bf5 16. Nb5 a6 17. d6 axb5 18. Bxf7+ Kxf7 19. Rxf5+ gxf5 20. Qh5+ Kf8 21. Bh6+ Bg7 22. Bxg7+ Kxg7 23. Qxe8 c4+ 24. Kh1 Qxd6 25. Re1 Qf8 (25...Ra6 26. Re7+ Kh6 27. Qf8+ Kg5 28. Qg8+) 26. Qe5+ Kg8 27. Qd5+ Kh8 28. Qe5+ 1/2-1/2
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I doubt very much that you can find anything that is both obvious (to the human eye) and new and strong, but a good candidate for the most drastically re-evaluated simple move might be 3..Nf6 against the Lopez.

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There are thousands of small evaluation changes, but there's only a few complete turnarounds busting main lines considered good for decades. Two examples come to my mind. First is correspondence game Voss-Zhak 2010, 30.Re6 winning, refuting the old main line of moscow gambit. Also 22.Kf1 from Giri-Shirov 2014 (found earlier in correspondence games) made old main line Sveshnikov almost lost for black. Both these recent games changed theory evaluations seriously.

  • Interesting, but by moves 22 and 30 I am not sure we're in the opening phase of the game. – Tony Ennis Aug 18 '17 at 11:45
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    @TonyEnnis The question was not about moves in the opening, but about moves (at any point during a game) that change the evaluation of an opening position. Also at GM level, moves 22 and 30 can well be considered opening phase in some lines. – user1583209 Aug 18 '17 at 12:07
  • "Opening lines" is in the title. – Tony Ennis Aug 18 '17 at 21:50
  • How about 26.Re7+ in the Vaisser-Bauer game? Isn't White winning then? – A. N. Other Aug 19 '17 at 8:39

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