One Hiccup

A KEY moment from this afternoon's round three of the 23rd 4NCL U2000/175 congress:

Black has just played 18...Qd8-d7 in Robert Clegg (1741/151) - Spanton (1927/168)

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The position is typical of ones arising from the Exchange Variation of the Queen's Gambit Declined. White's minority attack is progressing on the queenside, while Black usually hopes to create problems for the white king.
19.f4!?
Threatening to fork on f5 is very tempting. But the engines Stockfish10 and Komodo10 prefer 19.Nce2, with what Stockfish10 reckons is the upper hand for White, but Komodo10 evaluates as White being just slightly better.
19...Bxh3!
19...Qd6 was the 'safe' way to play, but better for White.
20.f5?
White probably should have played the 'obvious' 20.gxh3, when 20...Rxe3 gives a strong attack, although 20...Qxh3 is also possible. White could also have played 20.Bxg6!?, when 20...Rxe3 again seems good.
20...Rxe3
Also good was 20...Nh4.
21.Nce2
Or 21.fxg6 Rxg3, when Black emerges at least a pawn up and with a continuing attack.
21...Bxg2 22.Kxg2 Nh4+ 23.Kf2
The engines prefer 23.Kh1, but reckon Black is winning. Stockfish10's mainline runs 23...Ng4 24.Rf4 h5 25.f6 g6 26.Ra3 Rae8 27.Qc1 Qe6, when material is roughly level - Black has three pawns for a bishop - but Black's much better piece-coordination and White's endangered king mean Stockfish10 reckons Black is +7.25.
23...Ng4+ 24.Ke1 Qd6
With threats against b4 and g3.
25.Nh1 Rae8 26.Kd2 Qh2 27.Rae1 Nf3+ 28.Rxf3 Rxf3
Now Black has rook and three pawns for bishop and knight, but there was more drama to come.
29.Kc1 Rfe3 30.Qd2 Qh6 31.Kc2 Qd6 32.Rg1 h5 33.Nhg3 Nf6 34.Nf1 R3e7?
Careless. 34...Rxd3 wins easily, according to the engines, eg 35.Kxd3 Ne4, when material is again roughly even but Black's better piece-coordination and king safety are big factors.
35.Kd1?
Missing 35.Qh6, which draws, according to the engines, after 35...Rxe2+ 36.Bxe2 Kf8! 37.Qxg7+ Ke7 38.Rg6!? Qxb4 39.Qxf6+ Kd7, when White cannot stop Black from giving perpetual check.
35...Ne4
The danger to Black has gone, and the game finished:
36.Qe1 Qf6 37.Nfg3 Nxg3 38.Qxg3? Re3 0-1