There is, not surprisingly, no general consensus among critics as to what is the worst blunder ever made in a game of chess. After all, bad players make terrible moves every day and it is nothing special, so obviously a blunder must be worse the better the player is who makes it. And even Garry Kasparov might make a terrible move in a game of blitz chess over coffee with a friend, and slap his forehead, and reset the pieces in a hurry - so obviously, the more important the setting, the worse the blunder.
Bearing these factors in mind, I would like to submit 3 candidates for the worst chess blunders of all time.
3.
Bronstein - Botvinnik, World Championship Match, Moscow 1951 (6th Game)
This is about as bad as it gets. David Bronstein, by his own claim the world's strongest player at the time, is playing Mikhail Botvinnik, one of the greatest of all world chess champions, for the world title. He's a point ahead in the match, having won the previous game after four draws at the beginning, and he's playing well. The sixth game has been a great struggle, and in the diagrammed position Bronstein is a piece up, but he will have to sacrifice it back to prevent Black from promoting his passed pawn, with the result being a draw. He's seen how to do this - 1.Ne6+ Kg3 2.Nd4, and the game is a draw after 2...Kf2 3.Ka4 e2 4.Nxe2 Kxe2 5.Kxa5 Kd3 6.c5 Kxc3 7.Kb6 Kc4.
+----+----+----+----+----+----+----+----+
| | | | N | | | | |
+----+----+----+----+----+----+----+----+
| | p | | | | | | |
+----+----+----+----+----+----+----+----+
| | | | | | | | |
+----+----+----+----+----+----+----+----+
| p | | | | | | | |
+----+----+----+----+----+----+----+----+
| | | P | | | k | | |
+----+----+----+----+----+----+----+----+
| | K | P | | p | | | |
+----+----+----+----+----+----+----+----+
| | | | | | | | |
+----+----+----+----+----+----+----+----+
| | | | | | | | |
+----+----+----+----+----+----+----+----+
It's such an easy move that Bronstein should have played it straight away, but being a contemplative sort of person, and having lots of time left on his clock, he starts to think about where he went wrong in the game, and how he could have played it better. After a full 45 minutes of pondering and daydreaming, he decides it's time to make his move, completely forgets about 1.Ne6+, and plays 1.Kc2??. Botvinnik stares at him for a moment, probably unable to believe his luck, and plays 1...Kg3!, after which poor old Bronstein has to resign. The point is that after 1...Kf3, which is probably what Bronstein expected, 2.Ne6 still draws after 2...e2 3.Nd4+. However, after 1...Kg3 there is no check on d4, and so 2.Ne6 e2 3.Kd2 Kf2 or 3.Nd4 e1=Q both win for Black.
What makes this blunder even worse is the fact that Bronstein failed to take the world title from Botvinnik by half a point. Their match was tied at 12-12, and according to the rules at the time, a tie meant that the title holder retained the crown. Bronstein never seriously challenged for the world championship again, and must be absolutely sick of having to talk about this game in interviews.
2.
Von Popiel - Marco, Monte Carlo 1902
The next blunder is also very famous, but for different reasons. Played in 1902 between two strong masters, the position is as follows:
+----+----+----+----+----+----+----+----+
| | | | | | | | k |
+----+----+----+----+----+----+----+----+
| | b | | r | | | p | |
+----+----+----+----+----+----+----+----+
| p | | | | | | | p |
+----+----+----+----+----+----+----+----+
| | p | | | q | N | | |
+----+----+----+----+----+----+----+----+
| | | | b | P | | | |
+----+----+----+----+----+----+----+----+
| | | | Q | | | | |
+----+----+----+----+----+----+----+----+
| P | | | | | | P | P |
+----+----+----+----+----+----+----+----+
| | B | | R | | | | K |
+----+----+----+----+----+----+----+----+
Black (Marco) seems to be in trouble. His bishop on d4 is pinned, and cannot move without allowing the capture of his rook on d7. It's attacked three times and defended only twice, and once it's gone, the game is as good as over. Marco thinks carefully for a while, and then resigns, providing entertainment to many future chess enthusiasts. 1...Bg1! wins immediately for Black. Checkmate is threatened on h2 by the bishop and queen (2...Qxh2++) and at the same time White's queen on d3 is exposed to an attack from the rook on d7. White has to defend against the checkmate and capture the bishop on g1, after which Black captures the queen with an easy win. The lesson? Think twice before you give up.
+----+----+----+----+----+----+----+----+
| | | | | | | | k |
+----+----+----+----+----+----+----+----+
| | b | | r | | | p | |
+----+----+----+----+----+----+----+----+
| p | | | | | | | p |
+----+----+----+----+----+----+----+----+
| | p | | | q | N | | |
+----+----+----+----+----+----+----+----+
| | | | | P | | | |
+----+----+----+----+----+----+----+----+
| | | | Q | | | | |
+----+----+----+----+----+----+----+----+
| P | | | | | | P | P |
+----+----+----+----+----+----+----+----+
| | B | | R | | | b | K |
+----+----+----+----+----+----+----+----+
1.
Fischer - Bisguier, New York 1963
This counts as my top choice for the worst blunder ever made, because of its simplicity - no special knowledge of chess is required to understand it. Roberto Bisguier, a very strong grandmaster at the time, was tied for first place in the US Open Championships in 1963 with Bobby Fischer. Fischer was on top of the world at the time, and had already won the US Open 4 times, but Bisguier had been playing well, and fancied his chances in the game, which in fact is published and annotated (but without this anecdote) in Fischer's famous book, My 60 Memorable Games.
At some point, Bisguier hears something strange, and looks up to find Fischer snoring lightly, his eyes closed, slumped back in his chair. A glance at Fischer's clock shows that it won't be too long before he will lose on time. History does not record how long Bisguier grappled with his conscience, but he said after the game, "I made the worst move of my career."
"I woke up Bobby Fischer."
Fischer won the game a few moves later with a brilliant combination. A URL to the full game can be found in the notes below.
This writeup was inspired by one of my favourite chess books, The Complete Chess Addict.
Bronstein-Botvinnik, 1951: http://www.schachgeschichte.de/chess/games/wm51.htm
Fischer-Bisguier, 1963: http://chess.about.com/library/fischer60/blf47.htm | 
