A strategic mistake giving black a double attack on two weaknesses (Bc4 and pf4) with ..Qc7.

Compare the positions before and after the game sequence 12.f4 Qf7 13. Bb3 Nf4 14. Kh1 Ng6 15. Rc1 Qd8

< POSITION 1 >

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< POSITION 2 >

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Now the question is..
What does white gain, and at what cost?
(This is the question one must ask before the combo).

The black position.. exactly the same.
The white position..
a) The Bc4 is now on b3.
(still on the a2-g8 diagonal = no strategical change).

b) The QR is now on c1

c) The F pawn is missing /the F file is now open..

BUT..
White is minus another pawn. So the question..
Is Ra1-c1 (one tempo) and the open F file worth a pawn?

Black assessed it as NO, accepting the offer.
Why?
First, white's center is greatly weakened. He has a single e pawn against black's 3, thus it will require a piece sac to bust open the center. And without the F pawn white's hold on e5 vanishes.
Second, black has tactics to support the pawn capture, so white's "sac" lacks the vigor (tactical compensation) that follows a good gambit.
After ..Qc7 white is forced to make two unavoidable
defensive moves; Bb3 and Kh1.

< 13.Bb3 >

13. Qb3 Nxf4 14. Rac1 is no better since 14. ..Bc5! forces the liquidation of material after which the pawn deficite is more sorely felt.
<when down pawns, trade pawns not peices!>

< 14.Kh1 >
Necessary to avert the exchange of material and potential knight fork (the Nc3 is tied down to the defense of e2).

Instead of the blunder 12.f4? the more natural and preparatory < 12.Rc1 is much stronger >. Then if white is determined to move f4 the reply ..Qc7 can't be played. i.e.;
12.Rc1 b6 (or a6) 13.f4 Qc7?? 14.Nd5!

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