user8291: 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...

user11609: 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.

user13012: 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.

user-1: 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


8291: <p>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? </p>

<p>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...</p>
 11609: <p>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.</p>
 13012: <p>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.</p>
 -1: <p>Joe Gallagher says of 22... Kf8 in <em>Starting out: the king's indian</em>:</p>

<blockquote>
  <p>Vaisser relates how for many years this move was thought to lose because of some erroneous analysis.</p>
</blockquote>

<p>Of 25...Qf8 he writes:</p>

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

<pre><code>[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
</code></pre>
