Blunders can be very costly

Chess Checks

. The explanations then can range from miscalculation to oversight, but ‘blunders’ are part and parcel of many chess games even at the highest level. A momentary lapse in concentration due to tiredness or sometimes just not seeing the obvious move can change the entire complexion of the game.

In the game which follows, White appears to be holding an edge but Black equalizes and just when the game appears to be heading for a draw, Black blunders. The loss is immediate!
 White: Eckhard Schmittdiel (2505) – Black: Michall Feygin (2500) Master Tournament,  Dortmund, 1999
French Defence
 1.e4 e6 2.d4 d5
The French defence
 3.Nc3 Bb4 4.e5 c5 5.a3 Bxc3+ 6.bxc3 Qc7 7.Nf3
Most prefer the more dynamic continuation 7.Qg4 here
 7...b6 8.a4 Ne7 9.Bb5+ Bd7 10.0–0 Nbc6 11.dxc5
If 11.Ba3 Na5 12.Bxd7+ Qxd7
 11...bxc5 12.Ba3
White played a new move here. Previously played was 12.c4 a6 13.cxd5 axb5 14.d6 Qc8 15.dxe7 Nxe7
 12...Na5 13.Qd3 Rc8
If 13...0–0 14.Bxd7 (14.Bxc5 Qxc5 15.Bxd7 Ng6) 14...Qxd7 15.Bxc5 Rfc8
 14.Rfb1
Taking control of the open file
 14. ..0–0
If 14...c4 15.Qd2
 15.Bxc5 Qxc5
If 15...Bxb5 16.Rxb5 Rfe8 17.Bd6 Qd8 18.Bxe7 Rxe7 19.Ng5 f5 20.exf6 gxf6 21.Nxe6! Rxe6 22.Rxd5 Qe8
 16.Bxd7 Rb8
If 16...Rc7 17.Rb5
 17.Rb5 Rxb5
If 17...Qc7 18.Ng5 Ng6 19.Bxe6! fxe6 20.Nxe6 with advantage for White
 18.axb5 Qb6 19.Qd4
White looks better and hence the invitation to trade queens
 19. ..Rd8 20.Qb4
If 20.Qxb6 axb6
 20...Rxd7 21.Rxa5 Qb8
Rather passive! Black could have tried for some active play with 21...d4 22.Ra1 dxc3 23.Qxc3 Qxb5 with a balanced position
 22.g3 h6 23.Qa3 Ng6 24.Qa4
Trying for a small trap with the intention of pushing the ‘b’ pawn
 24. ..Rb7
If 24...Nxe5 25.Nxe5 Qxe5 26.b6 Rd8 27.bxa7 Ra8 28.Qc6 Qe1+ 29.Kg2 Qe4+ 30.Kf1 Qc4+ 31.Qxc4 dxc4 32.Ke2 Kf8 33.Ke3 Ke7 34.Kd4with White in a better position
 25.b6 Interesting advance!
 25. .. Rxb6 26.Rxa7 Qd8
If 26...Nxe5 27.Ra8 (27.Nxe5 Qxe5 28.Qe8+ Kh7 29.Qxf7 Qxc3) 27...Nxf3+ 28.Kg2 Ne5 29.Qa7
 27.h4 White decides to start proceedings on the King side with this pawn  advance
 27. ..Rb8 28.h5 Nf8
If 28...Ne7 29.Nd4 Nc8 30.Nc6
 29.Qf4 Swinging to the King side to target the vulnerable f7 pawn as his rook is also patrolling the seventh rank
 29. ..Qe8 30.c4 Ra8 Black would like to exchange this rook as it is dangerously placed. If 30...Rb4 31.Nd2
 31.Rc7
White is not interested in the exchange as he wants to retain the pressure
 31. ..Rc8 32.Rxc8 Qxc8 33.cxd5 exd5 34.Nd4
White is better placed now!
 34. ..Qc3
Worth a look is 34...Ne6
 35.Nf5 Ne6
If 35...Qe1+ 36.Kg2 Ne6 37.Qf3
 36.Qa4
White could also have explored the possibility of 36.Qg4
 36...Kh7 37.Ne7

Diagram 1

 37. ..Qxe5
A fatal blunder! In this equal position black missed a draw which he could get by playing 37...Nd4 38.Qe8 Qe1+ 39.Kg2 Qe4+
 38.Qe8
White infiltrates devastatingly into Black’s first rank to threaten checkmate
 38. ..Qe1+
More in desperation than anything else
 
39.Kg2 Nf4+ 40.gxf4 Qe4+ 41.Kg3 and Black resigned.

Diagram 2

Black to play and win New game
1. .. f3+ 2.Kg3
 If 2.Kg1 Qxf1+ 3.Kxf1 Rd1 checkmate
 2...  Qxf1 and Black wins.