Moves that cannot be played

One of the difficulties in sacrificing material is that it cuts out some of your options. Normally when you are material down you don't want to exchange off pieces, and this can make the defensive task for your opponent that much easier.
Here is a practical example from a game I played in a Ryder Gambit Thematic recently. In the Ryder Gambit White gives up one pawn and also offers a second. As I said previously, I don't think it is sound, and this game reveals some of the difficulties you can face. My opponent declined the offer of the second pawn, happy to be a single pawn up. Around move 10 I began to drift, and my opponent forced a couple of exchanges. However the big blunder occurred at move 15. Objectively the best move is 15.Nf5, preventing f5 (for the moment), but I felt this was I move I "couldn't" play, as it would allow yet another exchange. Instead I chose a second best move, and followed it up with an even worse move, dropping a piece.

Press,S - Kraakhoofd [D00]
BDG Thematic, 07.2008

1.d4 d5 2.e4 dxe4 3.Nc3 Nf6 4.f3 exf3 5.Qxf3 e6 6.Be3 Bb4 7.Bc4 Nc6 8.Nge2 0-0 9.0-0 e5 10.Ne4 Nxe4 11.Qxe4 Qe7 12.d5 Na5 13.Ng3 Nxc4 14.Qxc4 Bd6 (D)
15.Rae1 f5 16.Bd4 Qh4 0-1

Nonetheless, White can still do well with the gambit (in a practical sense) as Kraakhoofd won the event with 13.5/14 (6.5/7 with White), his only draw being in our second game (colours reversed), where even then he had a slight edge.