One error can make all the difference

 When everything is going right, a slight lapse or one bad move is enough to spoil the proceedings and bring about a downfall. Therefore it is important to calculate and think before every move even when the game appears to have been won!

In the game which follows, White is comfortable and has also launched an attack on his opponent’s King. However a blunder  on the 31st turn swings the tide in Black’s favour.
 White: Victor Mikhalevski (2545)- Black: Valery Loginov (2500) Beersheba-St Petersburg, 1998

King’s Indian defence
1.d4 Nf6 2.c4 g6 3.Nc3 Bg7
The King’s Indian defence
 4.e4 d6 5.h3 0–0 6.Bg5 Na6 7.Bd3 e5 8.d5 c6 9.Nge2 cxd5
Black can also play 9. ..Qb6
10.cxd5 Nc5 11.Bc2 a5 12.a3 a4 13.0–0
If 13.g4 Bd7 14.Kf1 Bd7
13. ..Bd7 14.g4.  Till now the players were playing in accordance with theory but now White plays a new move. If 14.Qd2 Qb6 15.Rab1 Rfc8 16.Ng3 Qd8 (If  16...Rc7 17.Be3) 17.Bh6 Bxh6 18.Qxh6 Qf8 19.Qe3 Be8  
14...h5 15.f3. Interestingly both players have pushed their King side pawns
15. ..b5. Black now acts on the queen side. If 15...Qb6 16.Kg2 Qxb2 17.Rb1 Qxa3 18.Bc1 with advantage for White
16.Na2. Perhaps 16.Qe1 appaers better! If 16.Kg2 Nxg4
16...Qb6 17.Kg2. Immediately getting out of the dangerous diagonal. If 17.Be3 hxg4 18.fxg4 Bxg4 19.hxg4 Nxg4 20.Bf2 leads to an unclear position
17...Rac8 18.Nb4. Worth a try is 18.Be3
18...Na6 19.Qd2
If 19.Nxa6 Qxa6 (19...Nh7 20.Qd2
19...Nxb4 20.axb4 Nh7! 21.Be7
If 21.Be3 Rxc2 and if 21.Bh4 Bh6 22.Qxh6 Rxc2 23.Rf2 Rxb2 24.gxh5 g5 25.Bg3 Qd8 with advantage for Black
21...Rfe8. If 21...Bh6 22.Qxh6 Rxc2 23.Rae1
22.Bh4 Bf6. An invitation to exchange bishops! Black should have tried 22... ..Bh6  23.Qxh6 Rxc2
23.Bxf6 Nxf6 24.Bd3 Kg7 25.Rf2
With the intention of doubling the rooks. If 25.g5 Nh7 26.f4 Qd8 (26...exf4 27.Qxf4 Rf8 28.e5 Rce8 29.exd6 Bxh3+) 27.h4 exf4 28.Qxf4 Qe7  
25...h4. If 25...Qd8 or 25...Nh7
26.Raf1. If 26.Qg5 Qd8 27.Qxh4 Nxg4 28.Qxd8 Ne3+ which works better for Black
26...Nh7 27.f4 27. .. exf4 28.Qxf4
If 28.Nxf4 Re7
28...Rf8. If 28...f6 29.g5
29.e5 dxe5 30.Qxe5+ Kg8
Diagram 1
31.Rf6. A blunder! White was clearly better and the correct continuation is 31.Qe7 Qb7 32.Nf4
31...Nxf6 32.Rxf6 Rce8
If 32...Rfe8 33.Qxe8+ Rxe8 34.Rxb6
33.Qg5
If 33.Qxe8 Qxf6 34.Qxd7 Qxb2 with advantage for Black
33...Rxe2+ A good counter sacrifice by Black. If 33...Qe3 34.Rxg6+ fxg6 35.Qxg6+ Kh8 36.Qh7 checkmate
34.Bxe2 Qd4 35.Rd6. White’s sense of danger has deserted him totally. The King is exposed and White should try to tighten his defence. He should have played 35.Rf2 Qxb2  36.Bxb5
35...Re8. Black is in the driver’s seat
36.Kf1 Qe4 37.Rxd7. If 37.Qd2 Qh1+ 38.Kf2 Qh2+
37...Qxe2+ 38.Kg1 Qe1+ and White resigned as 38...Qe1+ 39.Kg2 Qg3+ 40.Kh1 Re1 checkmate.
Diagram 2
It is advisable here to think and play rather than hurry with the obvious move!
White to play and win
1.Bh7+  If Qh7+Kf8 and things are no longer clear cut for White.
1.  ..Kh8 2.Bg6+ Kg8 3.Qh7+ Kf8 4.Qxf7 checkmate.