What's Wrong With That Move?

If you study an opening long enough, and play through enough master games with that opening, and play a lot of games with it yourself, you start to get a feel for which moves are accepatable or playable within the framework of that opening.  Such move are called "thematic" moves, or moves that follow the theme of the opening or position.  Consequently, some moves played by your opponent will strike you as "nonthematic", which is a fancy word for wrong.  When you encounter such a move, you have to make a decision about the move.  Is it A) a move that is known to be good, but with which you unfamiliar, or B) a move with which you are unfamiliar because it is a mistake no one has played before.  If A, then keep playing and learn something new.  If B, try to figure out why no one plays that move and refute it.  In the following game, I think Black's 12th move was a B move.

Well, it looks like I was right about 12...g6.  Why was I so confident that it was a mistake and not a good move?  Because I had studied and played this opening so much,  I just could not believe that a good move existed that I had not run across in my studies, either of this particular line or any similar lines.  How could I (and lots of GM's) have missed it?  We hadn't. It was a mistake. 
This goes to show you the value of studying an opening deeply and the value of playing over lots of master games in that opening.  You develop a feel for what should and should not be tried.  You get a sense of where to look on the board because you know how the pieces are related to each other.   You start to know on which squares pieces belong and don't belong.  It makes it a lot easier to answer the question "What's wrong with that move?"