Common Mistakes in the Opening Stage , Middlegame and Endgame:

Opening Stage common mistakes are:

Not developing pieces quickly enough;
Not developing pieces to good squares;
Making too many pawn moves, especially defensive ones like a2-a3, h2-h3, a7-a6 and h7-h6
when not necessary,
Waiting for the opponent to castle his king before castling ours;
Blocking the path of our pieces with other pieces;
Not playing for the initiative when the opponent has played unusual opening moves;
Not punishing the opponent’s mistakes;
Making routine developing moves on
auto-mode;
Not paying attention to the opponent’s moves and ideas;
Bringing out the queen too early; sometimes
Pawn-grabbing at the cost of king safety and development; and
Choosing the wrong opening.

Common Mistakes in the Middlegame are:

Playing passively with our pieces
Readily moving our pieces backwards at the first sign of aggression from the opponent;
Weakening the kingside pawn structure under the impression of attacking the opponent;
Being too eager to defend against imaginary threats;
Blindly exchanging pieces whenever it is offered or forcing unwanted exchanges to clarify the position;
Ignoring possible counterplay in the center or on the other flank for fear of an opponent’s attack on the side where we are weak;
Playing too fast when it is important to ponder over the position a little longer,
Thinking too much when it is not really necessary;
Not calculating variations thoroughly;
Not showing enough interest or inclination to find the best move in every position;
Having a lethargic approach, a casual approach, or a “let me handle it after the problem comes” approach;
Not paying attention to the opponent’s ideas and threats;
Not paying attention to the opponent’s potential mistakes and missing the moment to punish them;
Playing too many pawn moves in the name of activity, thus weakening the position:
Incorrect exchange of pawns and pieces;
Moving pieces aimlessly;
Failure to identify and exploit the opponent’s weaknesses
Poor time management;
Lack of basic plans, bad evaluation or misunderstanding of the position.

Common Mistakes in the Endgame are:

Playing for attack when the attack is non-existent or can be easily refuted;
Ignoring the improvement of the king;
Losing precious tempi by making too many pawn moves;
Exchanging the wrong piece at the wrong time;
Not watching out for our opponent’s ideas, especially counterplay possibilities;
Not being content with subtle improving moves when the situation warrants it, instead going for pseudo-active moves which spoil the position;
Lack of patience; and Not dedicating enough time to studying endings at home!

Students should memorize all the above common mistakes made by the players and avoid making them by themselves.