user25854: The PGN "Standard" (if you want to call it that, because it is very flawed and seems out of date), states:


First, if the moving pieces can be distinguished by their originating files, the originating file letter of the moving piece is
inserted immediately after the moving piece letter.
Second (when the first step fails), if the moving pieces can be distinguished by their originating ranks, the originating rank digit
of the moving piece is inserted immediately after the moving piece
letter.
Third (when both the first and the second steps fail), the two character square coordinate of the originating square of the moving
piece is inserted immediately after the moving piece letter.


What I want to know is this: what situation could give rise to the third case?
As I think this through, I cannot for the life of me think up a scenario that the first or second disambiguation rules wouldn't comprehensively cover.

user7297: @DM showed one possibility, which would also work with bishops (and on various other places on the board, with the three pieces at corners of a square). The only other option is with knights:
[FEN "8/1k6/8/8/5N2/8/1N3NK1/8 w - - 0 1"]

1. Nf2d3


user12629: Yes, it's possible for a piece to need both the rank and file to distinguish it, although it's rare. For example:
[FEN "8/1k6/8/8/5Q2/8/3Q1QK1/8 w - - 0 1"]

1.Qf2e3


25854: <p>The PGN &quot;Standard&quot; (if you want to call it that, because it is very flawed and seems out of date), states:</p>
<blockquote>
<ul>
<li>First, if the moving pieces can be distinguished by their originating files, the originating file letter of the moving piece is
inserted immediately after the moving piece letter.</li>
<li>Second (when the first step fails), if the moving pieces can be distinguished by their originating ranks, the originating rank digit
of the moving piece is inserted immediately after the moving piece
letter.</li>
<li>Third (when both the first and the second steps fail), the two character square coordinate of the originating square of the moving
piece is inserted immediately after the moving piece letter.</li>
</ul>
</blockquote>
<p>What I want to know is this: what situation could give rise to the third case?</p>
<p>As I think this through, I cannot for the life of me think up a scenario that the first or second disambiguation rules wouldn't comprehensively cover.</p>
 7297: <p>@DM showed one possibility, which would also work with bishops (and on various other places on the board, with the three pieces at corners of a square). The only other option is with knights:</p>
<pre><code>[FEN &quot;8/1k6/8/8/5N2/8/1N3NK1/8 w - - 0 1&quot;]

1. Nf2d3
</code></pre>
 12629: <p>Yes, it's possible for a piece to need both the rank and file to distinguish it, although it's rare. For example:</p>
<pre><code>[FEN &quot;8/1k6/8/8/5Q2/8/3Q1QK1/8 w - - 0 1&quot;]

1.Qf2e3
</code></pre>
