The Most Common Mistakes

1) Playing too fast because of overconfidence

• You think you are winning easily, or
• You think you are playing a lower-rated opponent.
  

2) Not recognizing the critical moment

• One definition of a critical move is when missing the best move can possibly change the expected outcome of the game.
  

3) Playing too fast because of carelessness
4) Being overcautious

• In trying to avoid small errors, they are making a bigger mistake by risking a loss on time or blundering in time trouble, and

• It is generally not worthwhile to try and differentiate the subtle distinctions between two equivalent moves. So use general principles to make non-critical moves rather quickly. A good start is to make each non-critical move using considerably less time than the average move of the time control.
  

5) Guarding instead of moving

• Guarding ties down other units to the defense,
• The opponent may be able to pile on additional attackers, and you either commit further resources to defensive functions or move the attacked piece anyway, and
• Guarding often allows the common tactic: removal of the guard.
  

6) Miscounting