Ponomariov made two bad moves involving his dark squared bishop: 14.Bf4 gives him an inferior position and 26.Bxf6 gives him a losing position.

This gives Ponomariev an unwelcome choice.

Ponomariev can either play an English opening reversed or else he can choose c3 and d4 which transposes into a king's pawn open game where White's queen on e2 becomes a liability instead of an asset.

Again, this is a computer engine "idea" against Closed Sicilian, it is played by engines and Fabiano, as usual, is well tuned into engine analyses. He is very well prepared.

Or Wilhelm Cohn.

Chigorin vs W Cohn, 1898

3.f4 is supposed to be a good response after 1.e4 e6 2.Qe2 e5, though. Lasker was more precise.

[This move is machine way to fight Closed Sicilian. And even though this is not a Closed Sicilian at this point it looks like that will be the transition into. Caruana knows about engine improvements, no doubt.]

8. d4

[new move, in the game Deviatkin-Jakov Geller, 2010 Russian Higher League a different idea was used involving d3, Na3-Nc4.]

14. Bf4??

[It is fair to say that white has not achieved anything in the opening but now he makes serious positional error allowing discovered attack on queen.

It is hard to rationalize such a strange "confidence" that will cost him an exchange.]

19. ...Bf5!?
20. Qxf5 Rxc3

[Caruana wants to reduce the material as soon as possible. Bishop pair is white's only hope but with every trade black is getting better. However with rooks exposed to bishop pair white has some chances.]

26. Bxf6?

[The best was to take what was offered:

26. Bxe8 Qxc3
27. Bb5 g5
28. Qg5 Re1+
29. Rxe1 Qxe1+
30. Kg2 Qe4+

and white loses pawn but has practical chances to try to save the ending. ]

26. ... Re1+ !

[Now white is clean exchange down, he will not have bishop pair any more and the game is effectively over.]

29. ...Rxe5

[Good practical decision. White position is completely hopeless. The rest of the game is characterized by desperate attempt of white to save hopeless position.]

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Absolutely mind-boggling blunder of white in 14th move. This is intentional blunder which makes it even more shocking. 2700+ player disregards one of the most elementary principles.