Breunor: I was looking on Stockfish for where the losing move is?
According to the computer, white is still solid after 17, up half a pawn:
1) +0.58 (28 ply) 17...Bf5 18.f3 Nf6 19.Re1 Rae8 20.Rxe8 Rxe8 21.a4 Qg6 22.axb5 Bd3 23.Qf2 axb5 24.Ne4 Bxe4 25.fxe4 Nxe4 26.Qf3 Bb8 27.Ra6 Nd6 28.Qf1 Ne4 29.Bc2 Nxg3 30.Bxg6 Nxf1 31.Bxh7+ Kxh7 32.Kxf1 Re6 33.Bxg5 Bxh2 34.Kg2 Rg6 35.Kxh2 Rxg5 36.Kh3 Kg7
The computer thinks it is a perpetual after 18 B x d5:
1) =0.00 (26 ply) 18...cxd5 19.Qe2 Qg6 20.Re3 Bf4 21.Re7 Bd6 22.Re3
I was really surprised to see that the losing move is the very natural looking
1) -1.95 (23 ply) 20...Bh3 21.Qe2 f6 22.Bd2 Bf4 23.Re1 Bxe3 24.fxe3 Kg7 25.Rc1 Bg4 26.Rf1 f5 27.Qf2 Bxf3 28.Qxf3 Qxf3 29.Rxf3 Rf7 30.Rf1 h6 31.Kf2 Rc7 32.h4 Kf6 33.hxg5+ hxg5 34.Rh1
Instead, white is just slightly down with Qg2:
1) -0.16 (23 ply) 20.Qg2 Bh3 21.Qf3 g4 22.Qf6 Rxe3 23.fxe3 Qg6 24.Qxg6+ fxg6 25.b4 Be7 26.Bb2 Bg5 27.Re1 Kg7 28.Bc1 Rc8 29.Bb2 Re8 30.Bc1 Bxe3+ 31.Kh1 Bxd2 32.Bxd2 Rxe1+ 33.Bxe1 Bf1 34.Kg1 Bc4 35.a3 Kf6 36.Kf2 Kf5 37.Ke3 h5 38.Bd2 g5 39.Kf2
It looks like the idea is to be able to move the queen to f3 after black plays Bh3
It surprised me - every instinct of mine was to try to get the white knight on d2 and especially the bishop on c1 into the game, but here it seems to be his undoing; I guess positioning his queen properly was more important.
Maybe Korchnoi fell apart then, because he should still be in it after Bf4, Stockfish shows two reasonable moves:
1) -1.18 (22 ply) 21.Rd3 Bc7 22.Bd2 Bxd3 23.Qxd3 Qg6 24.Qf1 h6 25.Ne1 Qh5 26.f3 Re6 27.Nc2 Rf6 28.Qg2 Qxf3 29.Qxf3 Rxf3 30.Nb4 Re8 31.Re1 Re4 32.Nxd5 Rxe1+ 33.Bxe1 Bd6
2) -1.32 (22 ply) 21.Rxe8 Rxe8 22.Ne1 Bh3 23.Ng2 Bxc1 24.Rxc1 Re2 25.Re1 Rxb2 26.Re8+ Kg7 27.Re5 Rxa2 28.Nf4 gxf4 29.Rxh5 Bxf1 30.Kxf1 f3 31.Ke1 Re2+ 32.Kf1 Rc2 33.Ke1 Rxc3 34.Rxd5 b4
I would think Korchnoi is OK at -1.2 against a player rated under 1800!
So we can view 21 g x f4 as a second losing move:
1) -2.70 (27 ply) 21...Bh3 22.Qxh3 Qxh3 23.Ne5 Qf5 24.fxg5 Qc2 25.Kg2 f6 26.gxf6 Rxf6 27.Rg3+ Kh8 28.Be3 Rg8 29.b4 Rf5 30.Re1 Qxa2 31.Rg1 Rxg3+ 32.hxg3 Qc2 33.Rh1 Rf6 34.Nd7 Qe4+ 35.Kh2 Rf5 36.Kg1 Qc2 37.Kg2 Qxc3 38.g4 Rf7 39.Ne5 Rf6
Although I would be very happy to see Korchnoi resign against me if I'm up only 2.7! Not sure I could take it home even with that lead against such a strong player!