What Happened??

Black plays reasonable chess in the following game, as does White - who slowly moves the game toward equality. Then, suddenly, the second player deliberately hangs a piece, and White's attack on the enemy King crashes through. Can Readers uncover an explanation (other than "chess blindness") for Black's 17th move?

Wall, Bill - Guest7132040
PlayChess.com, 2015

1.e4 e5 2.Nf3 Nc6 3.Bc4 Bc5 4.Bxf7+ 

4...Kxf7 5.Nxe5+ Nxe5 6.d4 Bxd4 7.Qxd4 d6 8.O-O

Or 8.Qd5+ followed by 9.O-O as in Wall,B - Guest344942, PlayChess.com, 2013 (1-0, 20).

8...Be6

8...Nf6 was seen in a number of previous games, for example:  Jerome,A - Shinkman,W, Iowa, 1876, (1/2 - 1/2, 42);  perrypawnpusher - JoseSoza, Chess.com, 2012 (0-1, 34);  Wall,B - Guest903719, PlayChess.com, 2013 (1-0, 47); and Wall,B - Guest1561957, PlayChess.com, 2014 (1-0, 25).

9.f4 Nc6 10.Qc3 Nf6 11.f5 Bd7 12.Qb3+ Kf8 13.Re1 Nd4 14.Qd3 c5

15.Bf4 Bb5 16.Qd1 Qb6 17.c3 Ba4

White can just take the Bishop. What am I missing? (If Black is planning 18...Qxb2 it would be well met by the obvious 19.Nd2.)

18.Qxa4 Nc6 19.Bxd6+ Kf7 20.Qc4+ Black resigned