When you trade pieces, calculating precisely if you will be up or equal material-wise is not always enough; you must correctly assess the resulting position.
Me! - Zlost https://lichess.org/Zg0STGWC/white#0
1.d4 Nf6
2.Nf3 c5
3.d5 g6
4.Nc3 Bg7
5.e4 0-0
6.Be2 a6
7.a4 d6
This is a classical Benoni structure. Black should aim for dynamic play. White tries to slow down Black's expansion on the queenside while aiming to play f4-e5. Black has to struggle positionally to find a decent square for his light-square bishop (ideally, he would not mind trading it for a knight).
8.h3 e6
9.dxe6 Bxe6
10.0-0 d5
Seemingly solving Black's positional problems: his light-square bishop is doing pretty well now, and there is no longer a backward pawn in d6.
11.e5 Nd7?!
This was an interesting moment. The engine recommends 11...Ne4. It seems that Black has to be faithful to the dynamic style typical of the Benoni. However, it is not obvious why Black should refrain from playing 11...Nd7, as he is indeed going to equalise the material pretty easily.
12.Bg5 Qc7
Now the massive simplification begins:
13.Nxd5 Bxd5
14.Qxd5 Nxe5
15.Nxe5 Qxe5
16.Qxe5 Bxe5
Done! And this should be equality. Or maybe not?
Well, the fact is that this is an open position where the bishop pair is clearly stronger than Black's minor pieces. It is true that White's b2-pawn is under attack, but the whole Black's queenside is weak. If you don't believe me check it with the engine (+1.8!).
17.Bf3 Nc6
18.c3 Rfe8
19.Rad1 f6
Black is hoping to stop the intrusion of White's rook along the 7th rank, but he does not have enough time for that, as White keeps attacking the weaknesses.
20.Be3 c4
21.Rd7 Re7?
22.Bxc6
1-0
A mistake in a difficult position (the evaluation was already +4 before the blunder...).
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