An opening disaster caused by playing a sideline which an opponent was able to prepare for, taken from Barnes, N (ECF:146) 0.5 - 0.5 Kjenner, S (ECF:154) (2016).
1. e4 e5 2. Nf3 Nc6 3. d4 exd4 4. Nxd4 is the Scotch Opening. 4...Qh4 is a sideline, but not a bad one. This was the move that White had expected Black to play.
5.Nb5 is not the most common reply but does give scope for Black to go wrong. The knight threatens to fork the king and rook by capturing on c7. 5...Bc5 replies with a stronger threat.
6.Qf3 is one way of protecting the f2 pawn and also sets a trap.
6...Ne5 looks strong as it threatens the white queen on f3 and so seems to be bringing another piece into the attack with tempo.
However, Ne5 was a serious error allowing White to win material.
7.Qf4! was the prepared trap. According to the computer the best option for Black is 7...Qxf2+ 8.Qxf2 Bxf2+ 9.Kxf2 and White has won a piece for a pawn. The game continuation was 7...Qxf4. White missed the best move here - 8.Nxc7+ as after 8...Kd8 9.Bxf4 the knight on c7 is safe as 9...Kxc7 10.Bxe5+ Kd8 11.Bxg7 and White has an even greater material advantage than if Black allows the knight to take the a8 rook.
White's preparation ran out on move 7 though and 8.Bxf4 Bd6 9.Nxd6 cxd6 leaves White much better (more than +2 according to the computer) but the advantage is not as clear cut as after 8.Nxc7+.