Sometimes you can find an answer by instinct. In this mainstream opening position, I had an opponent who played 5...d5? a move that clearly defies the logic of the position. He's opening up the center when behind in development, and willfully fixing himself with an isolated pawn that will be hard to defend. Alright, so 6.exd5 exd5, and then I sat there wondering how I could hurry up and win the isolani. I felt that his play was so illogical that I ought to be grabbing material immediately. 7.Bb5 Bd7 (maybe Nge7 can work? It holds the Nc6 and d5 pawn, but it is just so slow.) 8.Nxd5 Nxd4 9.Bxd7+ Qxd7 10.Qxd4 Rd8 and now, though I was still able to emerge with my pawn, the position had become at the very least, threatening. I suppose that my play was still about right, but the point is that these problems are often more difficult than they appear at first glance.
Here is a position from my most recent game:For readers who've seen some of my recent postings, yes, a large number of my games with both colors seems to be hedgehogs these days... At this point, I think it was time for black to play 10...Nbd7, probably to be followed by b7-b6 and a q-side fianchetto. Instead, my opponent played: 10...e5?! He wanted to activate his c8 bishop, and presumably hoped to treat the position like a mainline Kalashnikov. I knew that this had to be unsound, but how to prove it?
11 Nf5 Bxf5
12 exf5 Nc6
13 Re1 h6
14 f4 Rfe8
15 Qf2 Rad8
Clearly, there have been some less-than-perfect moves by both players to bring us to this position, and I think that white is still objectively for favor, but the point is that the whole endeavor has taken on a considerable note of risk. It would be easy for white to lose control of the position, dropping c4 or one of the f-pawns, and then black's two central pawns could really tell in an endgame.
Here I played 16.Nd5 and good things happened, but after 16...Nxd5 17.exd5 Nd4 18.Be3 Bf6, matters are far from clear, and the position is dangling close to a black advantage. The point is that white's pawns are unstable and rather over-extended.
I suppose that the moral of something like this is that you really can't afford to get too self-confident with your decisions just because you know that the opponent is trying something unkosher. If you don't have a clear instinct for the refutation, even very improbable plans can cause trouble.