In the following rather even position which resulted from a Ruy Lopez, my 1300ish rated opponent (Black) made a blunder of a rather common nature.
He played the hasty 13… Bg4? assuming that attacking the Q while developing the B had to be good. He saw 14.Qxg4 Nxg4 15.Bxd8 Rfxd8 results in an equal position and he also saw that it resulted in an equal exchange.
He missed that after 14.Bxf6 Bxf3 15.Bxd8 he was losing a piece so continued 15…Bxg2 16.Kxg2 Rfxd8 and had lost a piece for a P. Even after 14.Qg3 Bd7 15.f4 White has a fine game.
This seems to be a common mistake for players rated in this range. So what did he do wrong? He assumed White would have to move the attacked Q either by capturing the B or playing it to a ‘safe’ square. He failed to notice that the B was defended by only the N and that the removal of the guard by 14.Bxf6 left it hanging.
Had he noticed that it would have required running through the sequence to the end then counting up the material to see how things stood. This latter Part isn’t always so easy. GM Andy Soltis commented that it often turns out badly when your opponent has made the last capture, but that’s not always true. Black made the last capture in this sequence but he’d lost a piece.
Soltis gives two methods of how to review the material in your head. The first is rather clumsy in that you add up captured pieces as you go. The other method is to visualize the end position then count up the pieces. In this situation we would ‘see’ that for White the B, Q and g-Pawn have come off. For Black the N, B and Q. That leaves R’s and an extra White B on b3 still on the board.
Unfortunately there’s really no easy way of counting material and even GM’s have messed up. Black’s real problem was one of failing to notice the motifs listed in the following post: Thoughts on Development and Planning.