This mistake is a very common mistake and should be fixed. It is reacting to your opponent. If your opponent does a good move, and it hurts your position, then you shouldn't react to it. If you just keep a straight face and continue playing they may feel as their move wasn't so good. If you react, they will feel better about themselves. You should also not react to their move on the board. Sometimes, people blindly make a move just reacting to the opponent and not actually thinking about the position. In the position below, Lasker has just played 17...Qb6 avoiding the queen exchange.
Alexander Alekhine is white here and he saw a genius move, Qd6. Although it seems like a subtle move that does no damage it is very forcing and dangerous. In this position, Qb6 was a bad move by Lasker and caused the game to go downhill, but Alekhine, didn't just react to the move, he thought about the position and found a winning move.
The Second Mistake is, making assumptions. Most of the time, players assume that if they have taken an opponent's pieces, their opponent will recapture it, or if they sacrificed a piece, their opponent must capture which is not true at all. It is wrong to make assumptions, because your opponent may have something else up their sleeve. Look at this game as a good example played by paul Morphy, playing black
Morphy placed his knight on e4, threatening f2. White could just castle but he thought to capture the knight and Morphy will recapture then white castles. This leaves black with doubled isolated c pawns. 10.Bxe4, Qh4 Morphy didn't capture and he played an in-between move. Black is now attacking the f2 and the bishop. 11.Qe2, dex4 12. Be3 A bad move. Now Morphy increases the pressure, 12..Bg4 If 13.Qd2, ...Rd8 threatens the Queen and mate on d1. White resigned after 13.Qc4 Bxe3.
Important Points: 1. Do not assure your opponent will make a certain move. 2. If your opponent captures pieces or sacrifices, take a look at all options before capturing because you may have a better move.
Not Seeing The Opponent's Threat
Sometimes people are too busy with their own moves that they forget to think about their opponent's threats. You should remember to always look for your opponent's threats as it is very important that you consider their options and your defences.
We will be adding many more common mistake for you to learn from and avoid. Check the updates to see when.