user478: From the answer to my question How are PGN ambiguities handled?:

In the case of ambiguities (multiple pieces of the same type moving to
  the same square), the first appropriate disambiguating step of the
  three following steps is taken: ...

After reading that, a new ambiguity surged from that disambiguation, it is not clear what would happen if the pieces that look like they can move to a same square are all pinned except one piece, is then a need to disambiguate even if only one of those pieces can legally move in there?
Here is an illustrated example:
[FEN "4k3/8/8/8/8/8/8/q2NKN2 w - - 0 1"]

Should I use Ne3 or Nfe3?

user2001: Formally, both Nfe3 and Ne3 are valid notations in this situation. As far as I know, the convention is to write Ne3, because the Nd1 cannot move due to the pin Qa1-e1. But it is not impossible that someone would still scribble down Nfe3 in the score sheet and that is what you would find in your PGN file. Chess software would write Ne3 if you would present them with this position. If I would have this position, I would write Ne3 in my score sheet, because this follows the notation convention.

user672: It is not ambiguous for only one Ne3 is legal.

user478: just found this:

8.2.3.4: Disambiguation
Note that the above disambiguation is needed only to distinguish among
  moves of the same piece type to the same square; it is not used to
  distinguish among attacks of the same piece type to the same square. 
  An example of this would be a position with two white knights, one on
  square c3 and one on square g1 and a vacant square e2 with White to
  move.  Both knights attack square e2, and if both could legally move
  there, then a file disambiguation is needed; the (nonchecking) knight
  moves would be "Nce2" and "Nge2".  However, if the white king were at
  square e1 and a black bishop were at square b4 with a vacant square d2
  (thus an absolute pin of the white knight at square c3), then only one
  white knight (the one at square g1) could move to square e2: "Ne2".

http://www6.chessclub.com/help/PGN-spec

user954: Ne3 is perfectly fine, every normal pgn viewer knows legal moves and will perform it correctly.

user12: While it's not ambiguous (only Nfe3 is legal), it is better to be specific with Nfe3.  Successful software can deal with both Ne3 and Nfe3, but not every pgn parser is perfect, and some beginners will appreciate the additional guidance.

478: <p>From <a href="https://chess.stackexchange.com/a/1818/478">the answer</a> to my question <a href="https://chess.stackexchange.com/q/1817/478">How are PGN ambiguities handled?</a>:</p>

<blockquote>
  <p>In the case of ambiguities (multiple pieces of the same type moving to
  the same square), the first appropriate disambiguating step of the
  three following steps is taken: <strong>...</strong></p>
</blockquote>

<p>After reading that, a new ambiguity surged from that disambiguation, it is not clear what would happen if the pieces that look like they can move to a same square are all pinned except one piece, is then a need to disambiguate even if only one of those pieces can legally move in there?</p>

<p>Here is an illustrated example:</p>

<pre><code>[FEN "4k3/8/8/8/8/8/8/q2NKN2 w - - 0 1"]
</code></pre>

<p>Should I use <code>Ne3</code> or <code>Nfe3</code>?</p>
 2001: <p>Formally, both Nfe3 and Ne3 are valid notations in this situation. As far as I know, the convention is to write Ne3, because the Nd1 cannot move due to the pin Qa1-e1. But it is not impossible that someone would still scribble down Nfe3 in the score sheet and that is what you would find in your PGN file. Chess software would write Ne3 if you would present them with this position. If I would have this position, I would write Ne3 in my score sheet, because this follows the notation convention.</p>
 672: <p>It is not ambiguous for only one <code>Ne3</code> is legal.</p>
 478: <p>just found this:</p>

<blockquote>
  <p>8.2.3.4: Disambiguation</p>
  
  <p>Note that the above disambiguation is needed only to distinguish among
  moves of the same piece type to the same square; it is not used to
  distinguish among attacks of the same piece type to the same square. 
  An example of this would be a position with two white knights, one on
  square c3 and one on square g1 and a vacant square e2 with White to
  move.  Both knights attack square e2, and <strong>if both could legally move
  there, then a file disambiguation is needed</strong>; the (nonchecking) knight
  moves would be "Nce2" and "Nge2".  <strong>However</strong>, if the white king were at
  square e1 and a black bishop were at square b4 with a vacant square d2
  (thus <strong>an absolute pin</strong> of the white knight at square c3), <strong>then only one
  white knight (the one at square g1) could move</strong> to square e2: "Ne2".</p>
</blockquote>

<p><a href="http://www6.chessclub.com/help/PGN-spec" rel="noreferrer">http://www6.chessclub.com/help/PGN-spec</a></p>
 954: <p>Ne3 is perfectly fine, every normal pgn viewer knows legal moves and will perform it correctly.</p>
 12: <p>While it's not ambiguous (only <code>Nfe3</code> is legal), it is better to be specific with <code>Nfe3</code>.  Successful software can deal with both <code>Ne3</code> and <code>Nfe3</code>, but not every pgn parser is perfect, and some beginners will appreciate the additional guidance.</p>
