While it often happens that a single chess move throws away a game, it's not often that one move throws away an entire match, and even less often that a World Championship title evaporates at the same time. That is exactly what happened in game 11 of the Carlsen - Anand II match in Sochi (see Carlsen - Anand, Game 11 and Match for some background).
The diagram shows the position after Carlsen's 26th move. Until this point the game had been a tense Berlin variation in the Spanish Opening (aka Ruy Lopez) with the typical characteristics of the so-called Berlin Wall: no Queens, two Knights and a Bishop for White versus two Bishops and a Knight for Black, and a healthy 4-3 Pawn structure for White on the Kingside versus a crippled 4-3 structure for Black on the Queenside.
A few moves earlier, Black had succeeded in breaking open the Queenside, creating open lines and new targets for the remaining pieces. White's last move plans to bring his King into the center and Anand now went into a 22 minute think, his longest of the game.
Carlsen - Anand (game 11)
While Anand was thinking, the two commentators -- GM Peter Svidler and GM Ian Nepomniachtchi -- both superstrong, 2700+ Russian players, looked at many possibilities in this dynamic position. The main lines they discussed began 26...Be7, 26...Rdb8, and 26...Bg7, in rough order of preference.
After Anand played 26...Rdb8, they immediately launched into a discussion of 27.Rb1 Rb4, sacrificing the exchange.
IN: 'This is just strange. Why should we give an exchange for some shadowy compensation?' PS: 'It might be playable, but Black has no actual need to do this. [They look at a few moves.] Is this a winning attempt?' IN: 'It's not a losing attempt, at least. Since we're not losing on the spot we can try it. In this position, Black can never be worse.' PS: 'That's a large statement. I'm not sure I agree with that. [More moves] It looks interesting for Black, I have to say. Black now has everything under control, but it's a whole exchange. It's a risky way to continue.' IN: 'I don't think this is going to happen.'
Carlsen played 27.Ke4, 'allowing 27...Rb3', according to Svidler. Nepomniachtchi replied, 'That's a bold decision.' While they were looking at ...Rb3 and other ideas, Anand's 27...Rb4, appeared on the board. Talk about bold decisions!
IN: 'Wow!' PS: 'This is now very exciting. And we have a bit of an answer to the question we discussed earlier: Vishy does not feel like leaving everything until game 12. Black's position was not worse. He did not have to do that.' IN: 'Do you really think it's the best moment to sacrifice?' PS: 'It's a surprising moment, I have to say. You take, because you can't really not take. Magnus took about 20 seconds to take it. [...] This is a bit of a strange moment for the sacrifice. It was hanging in the air, the idea to play ...Rb4.' IN: 'I think Vishy could wait for a better moment.'
As we all know, Anand's idea didn't work and he resigned less than 20 moves later, thereby abandoning the match.
PS: 'Congratulations to Magnus for winning the title. This has been a very dramatic game. There is definitely some cause for regret for Vishy here.' [...] IN: 'Vishy went for some really unnecessary plan, with a completely unclear exchange sacrifice, and it just turned out that it was bad for him. [...] A strange decision took place today. I can't say if this exchange sacrifice was based on a miscalculation or some misunderstanding. I don't believe that Vishy can misunderstand chess. Even slightly.' PS: 'The only explanation is that he felt he must be doing something. He can't wait. He can't continue making normal quiet moves improving his position slightly. He finally settled on Rb8-b4, which backfired badly.'
At his press conference, Anand said,
I evaluated [the normal continuations] as equal. I can't say why I suddenly decided to go for the exchange sac. It was a bad gamble and I was punished. [...] Earlier I was still playing objectively. When we got to move 27 I took a nervous decision.
For the full game, see Magnus Carlsen vs Viswanathan Anand; Carlsen - Anand World Championship 2014 (game 11) on Chessgames.com.