Unless it's forced or is otherwise obviously the best move, I don't think that ...f6 should EVER be played by Black in the opening. Pushing the f-pawn two squares, instead of just one, is more often justified than moving it up only one square.
I'm not aware of any "good" defense in which Black effectively plays an early ...f6 other than in the conditions I mentioned above.
I suppose we could debate whether or not the Englund Gambit qualifies as a "good defense": 1 d4 e5 2 de Nc6 3 Nf3 f6.
Fellow named Mccreagor once drew Bobby Fischer with 3. Qe7 in a exhibition game once.
Horizon, Qh5+ fails to g6 since Nxg6 Qxe4+ Be2 (Kd1) and the simple Qxg6 puts black at a huge advantage.
Even with the correct 3rd move, the Damiano defense is simply bad since it gives white an unnecessarily huge lead in development.
Great trap that opens kingside attack
Can't you still check with Qh5+
Wouldn't that win a rook.
The reason that the domaino defense is bad is not Nxe5 but d4 or better Bc4. Those moves take advantage of the fact that black's position is made worse, not better by f6.
Knightassassin, good comment; I actually looked up that game before I wrote this when I was researching refutations to Nxe5. Let me bump that:
Fischer vs. McCreagor (1/2-1/2)
Horizoncantswim: bad continuation. Qh5+? ... g6! 2. Nxg6? ... Qxe4!+, which wins a knight for an equal game (black is down 2 pawns, queens are traded, and black's g-file has doubled pawns which preludes a capture of at least one of those [3 pawns = I knight])
Moopster, you are absolutely correct. If white chooses that continuation, though, black can use at least 5 pieces to control e5.
Bondiggity, you kind of missed the point. You don't need to immediately retake. D5 may be better than an immediate Qxe4. It would require a lot of coordination, but I wouldn't say it's damning to try.
Of all problems, the largest is that the king's file is not only open, but it is also shared by the Queen (careful for R-e1 pinning queen to king)
Also, consider this:
If black captures the Knight (...xNe4) and eventually does lose its rook to the Queen, black can trap the queen, though not for recapture. Instead, black can subjugate it to the corner where its attack potential is nullified. I believe Chigorin used this strategy to draw.
We all know this common trap: 1. e4 e5 2. Nf3 f3 (??) 3. Nxe5 fxNe5 4. Qh5+ g6 5. Qxe4+ and white wins black's rook. Despite what you all may be thinking, 2. ...f6 (the Damiano Defense) isn't the flaw, it's f6xNe4. So, what does black play? 3. Nxe4 ... Qe7! I would like to know what your thoughts are on this gambit. And remember, like all gambits, there is some risk involved (specifically, white may not take e5, in which case black may need to get a knight on g6 and over-defend e5).