Chess Opening Theory/1. d4/1...g5/2. Bxg5/2...f6

2. f6?
f6 seems like a relatively decent, if strange move by Black at first, kicking out the bishop and preparing for an advance to e5. However, this move weakens the king, as well as blocks development of the knight and the queen to f6. This can be abused by White with the move e4!, which makes the bishop poisoned, takes control of the center, and threatens to attack with the queen.

White always goes Qh5 after a few moves to either checkmate the king or force it to move away. This is not a good situation for Black to be in at all, with little development chances, being attacked, and a weak kingside.