Sometimes it might be possible to get off lightly with a slight mistake in a game of chess but blunders are unpardonable.  One bad move and the whole game collapses, like a pack of cards, undoing all the good work done earlier. A player blunders either due a lapse in concentration or he misjudges the position or miscalculates a continuation.

In the game which follows, the players stick to opening theory and the middle-game is fought on equal terms or maybe white does hold a slight edge.  He however goes haywire and commits a series of mistakes and then a big blunder with a wrong queen move which lands him in a hopelessly lost position.
White: Leonid Gofshtein (2517) – Black: Victor Mikhalevski (2531)
Ramat-Aviv ,1999.
Sicilian Defence.

1.e4 c5.
The Sicilian Defence.
2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 g6.
Opting for the sharp Dragon variation.
6.Be3 Bg7 7.Bc4 0–0 8.f3.
The equally sharp Yugoslav attack.
8. ..Nc6.
The players are rattling off the standard moves.
9.Qd2.
Intentions of queen side castling.
9. ..Bd7 10.Bb3 Rc8 11.0–0–0.
The players have castled on opposite sides and the attack will be on opposite flanks.
11. ..Ne5 12.Kb1.
He Could have tried 12.g4 or Bg5.
12. .. Nc4 13.Bxc4 Rxc4 14.g4 b5.
Both players want to attack on the king side but this move by black has been tried and to a large extent failed on previous occasions. Worth a try is 12. ..Qc7.
15.g5.
White also wants to walk headlong into enemy territory. He could have looked at the possibility of 15.Qd3.
15...Nh5 16.Nde2.
This is not a good move. White could have won a pawn with 16.Ndxb5 11/357; 16.Nce2 Qc7 17.c3 Rc8 18.Rc1 a5 19.Ng3 Nxg3 20.hxg3 Qb7 21.Nb3 Qc7 22.Bd4 e5 23.Be3 a4 24.Na1 d5 25.Qxd5 Be6 26.Qd2 a3 27.b3 Rxc3 28.Rxc3 Qxc3 29.Rc1 Qxd2 30.Rxc8+ Bxc8.  16...Be6.
Good move.
17.b3.
If 17.Nxb5 Bxb2 18.Kxb2 Qb8 19.Qa5 (19.Ned4 a6; 19.Nec3 a6) 19...a6! 20.Qxa6 Rb4+.
17...b4.
Black is sacrificing an exchange which is more or less along expected lines.
18.Na4.
White cannot accept the sacrificed exchange as  18.bxc4 bxc3 19.Nxc3 Qa5 20.Bd4 Bxc4 give advantage to Black. He could have however tried out 18.Nd5 Bxd5 19.exd5 Rh4 20.Bf2 Rh3 21.Ng1 ×h3;
18...Qa5.
Black’s pieces have now started targeting the King from close range while White’s pieces are nowhere near Black’s King.
 19.bxc4.
Another serious mistake by White which now allows Black to conjure up an attack out of nowhere.
He should have shut out the big diagonal with  19.Bd4 Rc7 with the idea Rfc8; 19.Nd4 Rc7 20.a3 Rb8  with Bd7 to follow.
19...Bxc4.
Things are getting difficult for White now with Black’s pieces posing dangerous threats.
20.Bd4 Qxa4 21.Nc1 Rc8.
Diagram 1
22.Qe3.
A big blunder which allows Black to immediately run away with the game. Even if he plays 22.Bxg7 Nxg7 black would still be in the driver’s seat.
22...Bxa2.
The  simple looking but most effective continuation in this position.
23.Nxa2.
If 23.Ka1 Be6+ 24.Kb1 Qxc2+ 25.Ka1 Rc5.
23...Qxc2+.
Getting too close for comfort and White is now hopelessly lost.
24.Ka1 b3 25.Rd2 b2+  and white resigned for if 25...b2+ 26.Bxb2 Bxb2 check-mate.
Diagram 2
White to play and win.
1.    1.Qxg8+
Sacrificing the queen to drag the King into a check-mating net.
1.    ..Kxg8 2.Rg2+ Kh8.
2.    If 2...Kf8 3.Rd8 check-mate. 
3.Rd8+ Bf8 4.Rxf8+ Rxf8 5.f7 check-mate.