You have to pay attention!!!
I have recently played a few online games and found myself doing completely idiotic mistakes. GM Gregory Serper has written a highly relevant article about how to lose a game in 10 moves or less. Now, I think it has been a while since I lost a game that quick. But the following two examples are just painful examples of how lack of attention can be a huge problem.
The following two examples are from an ongoing thematic tournament on the queen's gambit accepted. And coincidentally, the games are versus the same opponent, with a rating about 400 points lower than myself. Perhaps that's a part of the problem. Lower rating – expecting an easy win – lack of attention.
This first game is my first example of an idiotic move. See the diagram below, and try to find a proper response for white after 10...b6. I am sure you won't select the same move I did.
So I lost my knight. I did not pay attention. My lack of attention extended so far that it took me two more moves before I asked myself how I could be a piece behind. I was clearly not paying attention.
The next example deals with a somewhat more complex position. Black is up the exchange and has two passed pawns that are ready to march. But white is threatening the rook. How to deal with this threat? Try to find the winning combination for black.
I had calculated the continuation several moves ahead and had concluded that I was winning. I sacrificed the rook for a strong pawn advance that would lead to a new queen or the win of another piece. However, in my mind, I was too far ahead, and forgot that I had to take back on d8 before advancing the pawn. Once again, I did not pay attention.
Hindsight is always 20/20, but these games have taught me the importance of attention. In the first game, I managed to get the piece back, but blundered again later on and lost. In the second game, I was clearly winning, but blundered at a critical moment and found myself in a losing position.
So please learn from my mistakes: Pay attention!