It is very important to concentrate on every move. On every move, there is the possibility for you to blunder away a half-point or even a whole point.
This especially applies if you are winning. If you blunder in a lost position, you can give your opponent the chance of a really nice finish to the game. This is tiresome, but at least you can console yourself with the thought that the blunder didn’t cost you a point, as you were losing anyway.
But if you are winning and you blunder that can really hurt you.
It is possible to blunder even in the simplest looking positions.
Often players think that king and pawn endings are simple. But even they need concentration.
The following position looks very simple. White is to play and win. No need to think here, is there? This is just a king and pawn ending with a pawn on each side.
How many of us would play a6 without thinking? But it is wrong! Try to work out the correct move, and remember to always concentrate on every move.