user16333: I downloaded a bunch of games from Kingbase which included the below game.
[Event "FSIM November 2018"]
[Site "Budapest HUN"]
[Date "2018.11.06"]
[Round "4.4"]
[White "Farago, Sandor"]
[Black "Li, Hong"]
[Result "1-0"]
[WhiteElo "2216"]
[BlackElo "2337"]
[ECO "B80"]
[EventDate "2018.11.03"]
[FEN ""]

1.e4 c5 2.Nf3 d6 3.d4 Nf6 4.Nc3 cxd4 5.Nxd4 a6 6.f3 e6 7.Be3 b5 8.a3 Bb7 
9.g4 h6 10.h4 Nc6 11.Nxc6 Bxc6 12.Rg1 Nd7 13.g5 hxg5 14.hxg5 Be7 15.Qd2 
Rh7 16.O-O-O Nf8 17.f4 Qa5 18.f5 e5 19. null Nd7 20.Kb1 Rb8 21.Nd5 Qxd2 22.
Rxd2 Bxd5 23.Rxd5 Rh4 24.Bd3 Rh3 25.Ba7 Rb7 26.Bf2 Rh2 27.Be3 Rh3 28.Re1 
Nb8 29.f6 gxf6 30.gxf6 Bxf6 31.Rxd6 Bg7 32.Rd5 Nc6 33.Bd2 f6 34.Rg1 1-0

I'm confused about the notation in move 19 . I couldn't find any reference on how to interpret it. According to another source the tournament ended after move 18.
At first I thought maybe the time ran out and they decided to finish the game just for fun, But then why didn't White make the next move, and not black?
Different PGN replayers display this game differently. Gnome-chess refuses to display the game with move 19, while chess.com analysis interprets it as an empty move.

user9192: I have an alternative hypothesis. When I looked at the game before move 19, it seemed to me that White has a big advantage by pushing f6. Indeed, Stockfish (10+) agrees. It could be that Black resigned after move 18, but one or both of them was curious as to whether Black could save the game if given an extra move, so they continued playing after skipping White's turn.
One way to block f6 is indeed Nd7, after which White's threat of f6 is completely nullified and White no longer has a big advantage, in agreement with Stockfish, and now is better off playing Kb1 to preemptively protect against b4.
Incidentally, Stockfish (at least at depth 20) thinks Black's best extra move should be not Nd7 but b4.
Although it is hard to believe that illegal moves after a game could make it into a database, it is also a bit hard to believe that White moved Be2 (a rather poor move) and that Black would just resign after White's move 34, since there is no clear advantage for White at that point (except being 1 pawn up).
I suppose the best way to know for sure is to ask the players themselves.

user19877: I do not think that the game just ended there. It is more likely that the second source simply could not handle the null move.
This is a bit unusual that it would be so through, but looking at the subsequent moves, we can rule out almost every move there, and figure out what had to have been played.
The available moves on move 19 are: Kb1, Qd2 moves, Rd1, Be3 moves, Bf1 moves, Rg1 moves, and the one of the k-side pawns move to the sixth rank. It is doubtful that either the a-pawn, or b-pawn moved as both players are Masters, and those are horrible moves.

We know that it was not Kb1 since that moved on move 20.
We know that the Qd2 did not move since it was exchanged on d2 on move 21.
We know it was not the Rd1 since it recaptured the queen on move 22.
It is highly doubtful that the Be3 moved since f2 would have been the only square, and it later moved when attacked on move 24...Rh3.
This is my guess that the B went to e2 since it is literally the only move left.
It cannot be that the Rg1 moved since it later moved Re1 on move 28.
It cannot be the k-side pawns moving since they got traded on move 29.

The last possibility is that there was an error in the scoresheet prior to the null move, and whoever was recording this just moved on from there.
On the First Saturday Chess Tournament web site, where this was played, the game viewer lists this scoresheet after this move as "unreadable".
Update: Based on Evargalo's Qd4 move hypothesis, that probably should be the accepted answer as it makes great sense positionally.

user14079: Best guess: the mistake occured before, on move 15
Entry errors are frequent when games are entered into a software, especially if they are not entered by one of the players.
Here the operator must have made an error before move 19, reached an anomaly, and couldn't solve the mystery (or didn't have time to try, there are other games to be saved). In such occurence, entering a null move solves the issue, even if it makes some future users wonder what exactly happened during the game. 
Only one of the players, or someone with access to the handwritten gamescore, could answer with 100% certainty, so I cannot be totally affirmative, but I am pretty sure the error occured on move 15. This must be how the game went:
[Event "FSIM November 2018"]
[Site "Budapest HUN"]
[Date "2018.11.06"]
[Round "4.4"]
[White "Farago, Sandor"]
[Black "Li, Hong"]
[Result "1-0"]
[WhiteElo "2216"]
[BlackElo "2337"]
[ECO "B80"]
[EventDate "2018.11.03"]
[FEN ""]

1.e4 c5 2.Nf3 d6 3.d4 Nf6 4.Nc3 cxd4 5.Nxd4 a6 6.f3 e6 7.Be3 b5 8.a3 Bb7 
9.g4 h6 10.h4 Nc6 11.Nxc6 Bxc6 12.Rg1 Nd7 13.g5 hxg5 14.hxg5 Be7 15.Qd4 
Rh7 16.O-O-O Nf8 17.f4 Qa5 18.f5 e5 19. Qd2 Nd7 20.Kb1 Rb8 21.Nd5 Qxd2 22.
Rxd2 Bxd5 23.Rxd5 Rh4 24.Bd3 Rh3 25.Ba7 Rb7 26.Bf2 Rh2 27.Be3 Rh3 28.Re1 
Nb8 29.f6 gxf6 30.gxf6 Bxf6 31.Rxd6 Bg7 32.Rd5 Nc6 33.Bd2 f6 34.Rg1 1-0


15.Qd4, instead of 15.Qd2, is a very logical move, centralizing the queen and hitting the Pg7 left unprotected by Black's last move, 14...Bf8-e7. It is also a typical move when a Nd4-Nc6 exchange occurs in the Najdorf with the wQ still on d1.
It doesn't take a lot of stretch for an handwritten "Qd4" to be mistaken for "Qd2" (another plausible move) by the operator. 
Then the operator won't meet any incoherence before move 19, when he finds 19.Qd2 written on the scoresheet but, puzzlingly, the queen already stands on d2. If the explanation doesn't jump to his eyes, using the null move feature of the software (most probably Chessbase) is the simplest way to deal with it. I have done that often myself when entering kids' games whose scoresheets I couldn't decipher.
By the way, 19.Qd2 is the obvious choice if the queen comes from d4, attacked by Black last move, 18...e6-e5. The whole game looks very coherent in this version (but that Black is positionnally outplayed pretty fast, with 22...Bxd5? a very suspect choice).
The most important argument in favor of 15.Qd4, in my opinion, is that it gives an explanation for the otherwise mysterious move 15...Rh7 played by Black: he simply protected his pawn. If 15.Qd2 had been played, 15...Rh7 would be a very, very weird decision (instead of any of the logical developing moves 15...Qc7, 15...Nf8, 15...Rc8, 15...Nb6, 15...g6 or 15....Qa5).


16333: <p>I downloaded a bunch of games from <a href="http://www.kingbase-chess.net/" rel="nofollow noreferrer">Kingbase</a> which included the below game.</p>

<pre><code>[Event "FSIM November 2018"]
[Site "Budapest HUN"]
[Date "2018.11.06"]
[Round "4.4"]
[White "Farago, Sandor"]
[Black "Li, Hong"]
[Result "1-0"]
[WhiteElo "2216"]
[BlackElo "2337"]
[ECO "B80"]
[EventDate "2018.11.03"]
[FEN ""]

1.e4 c5 2.Nf3 d6 3.d4 Nf6 4.Nc3 cxd4 5.Nxd4 a6 6.f3 e6 7.Be3 b5 8.a3 Bb7 
9.g4 h6 10.h4 Nc6 11.Nxc6 Bxc6 12.Rg1 Nd7 13.g5 hxg5 14.hxg5 Be7 15.Qd2 
Rh7 16.O-O-O Nf8 17.f4 Qa5 18.f5 e5 19. null Nd7 20.Kb1 Rb8 21.Nd5 Qxd2 22.
Rxd2 Bxd5 23.Rxd5 Rh4 24.Bd3 Rh3 25.Ba7 Rb7 26.Bf2 Rh2 27.Be3 Rh3 28.Re1 
Nb8 29.f6 gxf6 30.gxf6 Bxf6 31.Rxd6 Bg7 32.Rd5 Nc6 33.Bd2 f6 34.Rg1 1-0
</code></pre>

<p>I'm confused about the notation in move 19 . I couldn't find any reference on how to interpret it. According to <a href="https://www.365chess.com/tournaments/FSIM_November_2018_2018/43020" rel="nofollow noreferrer">another source</a> the tournament ended after move 18.</p>

<p>At first I thought maybe the time ran out and they decided to finish the game just for fun, But then why didn't White make the next move, and not black?</p>

<p>Different PGN replayers display this game differently. Gnome-chess refuses to display the game with move 19, while <a href="https://www.chess.com/analysis" rel="nofollow noreferrer">chess.com analysis</a> interprets it as an empty move.</p>
 9192: <p>I have an alternative hypothesis. When I looked at the game before move 19, it seemed to me that White has a big advantage by pushing <code>f6</code>. Indeed, <a href="https://lichess.org/analysis/standard/r3kn2/4bppr/p1bp4/qp2pPP1/4P3/P1N1B3/1PPQ4/2KR1BR1_w_q_-_0_19" rel="nofollow noreferrer">Stockfish (10+) agrees</a>. It could be that Black resigned after move 18, but one or both of them was curious as to whether Black could save the game if given an extra move, so they continued playing after skipping White's turn.</p>

<p>One way to block <code>f6</code> is indeed <code>Nd7</code>, after which White's threat of <code>f6</code> is completely nullified and White no longer has a big advantage, <a href="https://lichess.org/analysis/standard/r3k3/3nbppr/p1bp4/qp2pPP1/4P3/P1N1B3/1PPQ4/2KR1BR1_w_q_-_1_20" rel="nofollow noreferrer">in agreement with Stockfish</a>, and now is better off playing <code>Kb1</code> to preemptively protect against <code>b4</code>.</p>

<p>Incidentally, <a href="https://lichess.org/analysis/standard/r3kn2/4bppr/p1bp4/qp2pPP1/4P3/P1N1B3/1PPQ4/2KR1BR1_b_q_-_0_19" rel="nofollow noreferrer">Stockfish (at least at depth 20) thinks</a> Black's best extra move should be not <code>Nd7</code> but <code>b4</code>.</p>

<p>Although it is hard to believe that illegal moves after a game could make it into a database, it is also a bit hard to believe that White moved <code>Be2</code> (a rather poor move) and that Black would just resign after White's move 34, since there is no clear advantage for White at that point (except being 1 pawn up).</p>

<p>I suppose the best way to know for sure is to ask the players themselves.</p>
 19877: <p>I do not think that the game just ended there. It is more likely that the second source simply could not handle the null move.</p>

<p>This is a bit unusual that it would be so through, but looking at the subsequent moves, we can rule out almost every move there, and figure out what had to have been played.</p>

<p>The available moves on move 19 are: Kb1, Qd2 moves, Rd1, Be3 moves, Bf1 moves, Rg1 moves, and the one of the k-side pawns move to the sixth rank. It is doubtful that either the a-pawn, or b-pawn moved as both players are Masters, and those are horrible moves.</p>

<ol>
<li>We know that it was not Kb1 since that moved on move 20.</li>
<li>We know that the Qd2 did not move since it was exchanged on d2 on move 21.</li>
<li>We know it was not the Rd1 since it recaptured the queen on move 22.</li>
<li>It is highly doubtful that the Be3 moved since f2 would have been the only square, and it later moved when attacked on move 24...Rh3.</li>
<li>This is my guess that the B went to e2 since it is literally the only move left.</li>
<li>It cannot be that the Rg1 moved since it later moved Re1 on move 28.</li>
<li>It cannot be the k-side pawns moving since they got traded on move 29.</li>
</ol>

<p>The last possibility is that there was an error in the scoresheet prior to the null move, and whoever was recording this just moved on from there.</p>

<p>On the First Saturday Chess Tournament web site, where this was played, <a href="http://firstsaturday.hu/1811/CB/im/games/dgt_chesstheatre.htm" rel="nofollow noreferrer">the game viewer lists this scoresheet after this move as "unreadable".</a></p>

<p>Update: Based on Evargalo's Qd4 move hypothesis, that probably should be the accepted answer as it makes great sense positionally.</p>
 14079: <p><strong>Best guess: the mistake occured before, on move 15</strong></p>

<p>Entry errors are frequent when games are entered into a software, especially if they are not entered by one of the players.</p>

<p>Here the operator must have made an error <em>before</em> move 19, reached an anomaly, and couldn't solve the mystery (or didn't have time to try, there are other games to be saved). In such occurence, entering a null move solves the issue, even if it makes some future users wonder what exactly happened during the game. </p>

<p>Only one of the players, or someone with access to the handwritten gamescore, could answer with 100% certainty, so I cannot be totally affirmative, but I am pretty sure the error occured on move 15. This must be how the game went:</p>

<pre><code>[Event "FSIM November 2018"]
[Site "Budapest HUN"]
[Date "2018.11.06"]
[Round "4.4"]
[White "Farago, Sandor"]
[Black "Li, Hong"]
[Result "1-0"]
[WhiteElo "2216"]
[BlackElo "2337"]
[ECO "B80"]
[EventDate "2018.11.03"]
[FEN ""]

1.e4 c5 2.Nf3 d6 3.d4 Nf6 4.Nc3 cxd4 5.Nxd4 a6 6.f3 e6 7.Be3 b5 8.a3 Bb7 
9.g4 h6 10.h4 Nc6 11.Nxc6 Bxc6 12.Rg1 Nd7 13.g5 hxg5 14.hxg5 Be7 15.Qd4 
Rh7 16.O-O-O Nf8 17.f4 Qa5 18.f5 e5 19. Qd2 Nd7 20.Kb1 Rb8 21.Nd5 Qxd2 22.
Rxd2 Bxd5 23.Rxd5 Rh4 24.Bd3 Rh3 25.Ba7 Rb7 26.Bf2 Rh2 27.Be3 Rh3 28.Re1 
Nb8 29.f6 gxf6 30.gxf6 Bxf6 31.Rxd6 Bg7 32.Rd5 Nc6 33.Bd2 f6 34.Rg1 1-0
</code></pre>

<ul>
<li><p><strong>15.Qd4</strong>, instead of 15.Qd2, is a very logical move, centralizing the queen and hitting the Pg7 left unprotected by Black's last move, 14...Bf8-e7. It is also a typical move when a Nd4-Nc6 exchange occurs in the Najdorf with the wQ still on d1.</p></li>
<li><p>It doesn't take a lot of stretch for an handwritten "Qd4" to be mistaken for "Qd2" (another plausible move) by the operator. </p></li>
<li><p>Then the operator won't meet any incoherence before move 19, when he finds 19.Qd2 written on the scoresheet but, puzzlingly, the queen already stands on d2. If the explanation doesn't jump to his eyes, using the <em>null move</em> feature of the software (most probably Chessbase) is the simplest way to deal with it. I have done that often myself when entering kids' games whose scoresheets I couldn't decipher.</p></li>
<li><p>By the way, 19.Qd2 is the obvious choice if the queen comes from d4, attacked by Black last move, 18...e6-e5. The whole game looks very coherent in this version (but that Black is positionnally outplayed pretty fast, with 22...Bxd5? a very suspect choice).</p></li>
<li><p>The most important argument in favor of 15.Qd4, in my opinion, is that it gives an explanation for the otherwise mysterious move 15...Rh7 played by Black: he simply protected his pawn. If 15.Qd2 had been played, 15...Rh7 would be a very, very weird decision (instead of any of the logical developing moves 15...Qc7, 15...Nf8, 15...Rc8, 15...Nb6, 15...g6 or 15....Qa5).</p></li>
</ul>
