The "Winning" Move
The diagram shows the final few moves from my game in the DHLC Slow League last night. Seeing that the bishop on e6 was pinned, I played 22.Nf5, since the double threat of capturing the queen and checkmate looked too good to be true. I had only calculated one sensible reply: after blocking the mate with Qf6, White trades queens and then wins the exchange with the knight fork on e7 (first variation in diagram). In the game, my opponent completely missed the mate threat and only moved the queen to safety on e8. But we both missed another sensible way of saving the queen and protecting g7: after Qg5, White is on his heels. In order to save his knight, he loses two pawns (and likely the game) in the aftermath! (second variation in diagram)
Simple lesson learned: once you find that your opponent's expected reply to your threat doesn't work, spend a moment trying to find the unexpected reply!