user1811: In a given situation, can we assume that a chess engine is tactically perfect - meaning that it can find all possible tactics which are there in that position. Is it possible for the best of modern chess engines to miss a tactic in a given position? Has this ever been observed in any recent game?
EDIT: @Henry Keiter: For the purpose of this question, I would define tactic as a combination or sequence of moves which wins material(if playing the winning side), or a combination or sequence of moves which defends a difficult position successfully (meaning preventing a loss of material which seems imminent). I know that the general definition is broader but for the purpose of this question the meaning can be taken as defined above.
EDIT 2: The reason I am asking this question is I am actually curious if a chess engine really does find all the possible tactics in a given position, or if it just manages to find many more tactics than a human being would find.

user2429: 
Is it possible for the best of modern chess engines to miss a tactic in a given position?

This question boils down to

Is it possible for the best of modern chess engines to have a bug in their calculation or evaluation module? 

Technically, yes. Sometimes, there are bugs which are extremely hard to detect unless they first occur and are caught. But it seems rather unlikely that such bugs will exist in the best engines which I am sure go through extremely thorough testing; that's one of the reasons why they are the best! :)

user2001: The quality and depth of an engine's calculation depends on the

Performance of the machine the engine is running on
Quality of its algorithms in terms of calculation and performance
Time span allowed for calculating the position

It is generally considered that tactics (i.e. combinations that lead to winning material or delivering checkmate) are the engines' speciality. There is a double difficulty of calculating variations. First, to find the best sequence of moves for both sides. Second, to evaluate the final positions after the combination is completed. Todays state of the art engines are excellent at both. I would say that

Todays best engines will not miss a single tactic.

Most probably, there are "tactical benchmark data sets" of positions that engines constantly are tested against. I don't recall any cases where engines missed tactics. But by going through the game databases, especially from engine versus engine tournaments, there should be examples of one engine calculating better than its opponent engine.

user167: In objective terms, the game of chess is nothing but tactics. So the question of tactically perfect play is just the question of optimal play. Given sufficient time for any given position, then yes, any competently-built engine would indeed be tactically perfect, because it could search the entire game tree from the position and play optimally.
But here's the thing: in general, the problem of determining optimal play is computationally unfeasible, so we don't even have a way to determine in general whether a tactic has been missed or not. For all we know, 1.a3 from the start position is the only winning move in a mate-in-80 tactics puzzle. If that is the case, then any time an engine chooses to play another move it is missing a tactic. We have no way to prove that isn't the case.

1811: <p>In a given situation, can we assume that a chess engine is tactically perfect - meaning that it can find all possible tactics which are there in that position. Is it possible for the best of modern chess engines to miss a tactic in a given position? Has this ever been observed in any recent game?</p>

<p>EDIT: @Henry Keiter: For the purpose of this question, I would define tactic as a combination or sequence of moves which wins material(if playing the winning side), or a combination or sequence of moves which defends a difficult position successfully (meaning preventing a loss of material which seems imminent). I know that the general definition is broader but for the purpose of this question the meaning can be taken as defined above.</p>

<p>EDIT 2: The reason I am asking this question is I am actually curious if a chess engine really does find all the possible tactics in a given position, or if it just manages to find many more tactics than a human being would find.</p>
 2429: <blockquote>
  <p>Is it possible for the <strong>best</strong> of modern chess engines to miss a tactic in a given position?</p>
</blockquote>

<p>This question boils down to</p>

<blockquote>
  <p>Is it possible for the best of modern chess engines to have a bug in their calculation or evaluation module? </p>
</blockquote>

<p>Technically, yes. Sometimes, there are bugs which are extremely hard to detect unless they first occur and are caught. But it seems rather <strong>unlikely</strong> that such bugs will exist in the <strong>best</strong> engines which I am sure go through extremely thorough testing; that's one of the reasons why they are the best! :)</p>
 2001: <p>The quality and depth of an engine's calculation depends on the</p>

<ol>
<li>Performance of the machine the engine is running on</li>
<li>Quality of its algorithms in terms of calculation and performance</li>
<li>Time span allowed for calculating the position</li>
</ol>

<p>It is generally considered that tactics (i.e. combinations that lead to winning material or delivering checkmate) are the engines' speciality. There is a double difficulty of calculating variations. First, to find the best sequence of moves for both sides. Second, to evaluate the final positions after the combination is completed. Todays state of the art engines are excellent at both. I would say that</p>

<blockquote>
  <p>Todays best engines will not miss a single tactic.</p>
</blockquote>

<p>Most probably, there are "tactical benchmark data sets" of positions that engines constantly are tested against. I don't recall any cases where engines missed tactics. But by going through the game databases, especially from engine versus engine tournaments, there should be examples of one engine calculating better than its opponent engine.</p>
 167: <p>In objective terms, the game of chess is nothing but tactics. So the question of tactically perfect play is just the question of optimal play. <em>Given sufficient time for any given position</em>, then yes, any competently-built engine would indeed be tactically perfect, because it could search the entire game tree from the position and play optimally.</p>

<p>But here's the thing: in general, the problem of determining optimal play is computationally unfeasible, so we don't even have a way to determine in general whether a tactic has been missed or not. <strong>For all we know, <code>1.a3</code> from the start position is the only winning move in a mate-in-80 tactics puzzle.</strong> If that is the case, then any time an engine chooses to play another move it is missing a tactic. We have no way to prove that isn't the case.</p>
