This position looks like the usual setup for a back rank mate, starting with a sacrifice to draw the Black defender of the rank forward. White has a queen and bishop both aimed at g7, and a rook available to follow up; so the natural idea is 1. Qxg7+, QxQ, and now the rook mates with Rf8. What’s wrong with this idea? Never forget to consider possible interpositions, even when the attacking pieces are as snugly arranged against the king as we find here. Black can reply to Rf8+ with Bg8, and now White will be losing a piece for a pawn.