Alertness in chess is very important. This game gives one reason why. Kienenger-MRoss, Munich, 1941:1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 a6 6.g3 b5 7.Bg2 Bb7 [Opposing bishops like this always requires special alertness.]8.0–0 Nbd7 9.Re1 e6? [Black needed to bite the bullet with 9...e5 10.Nf5 Qc7]10.e5! Bxg2 11.exf6 [That's right! White didn't have to retake.]11...Bb7 12.fxg7 Bxg7 13.Nf5 [Another winning way was 13.Nxe6 fxe6 14.Rxe6+ Kf7 15.Rxd6 Bc8 16.Ne4 h6 17.Qd5+]13...Bf8 (See Diagram) and now Black is appropriately punished for the error of his ways.
It finished with 14.Rxe6+ Black Resigned as after 14...fxe6 [or 14...Be7 15.Nxd6+ Kf8 16.Bh6+ Kg8 17.Qg4+ Bg5 18.Re8+ Nf8 19.Rxf8+ Qxf8 20.Qxg5+ Qg7 21.Qxg7#]15.Qh5#