user15543: Here's a question that's a little off the usual format, but problem questions like this have been done before and are on topic, so here we go!
Question: Can you find the mysterious mate in one? The trick is not board rotation, removing a piece, retracting any moves, completing a move, or an illegal move-there is but one solution. Have fun solving!
 [Title "White checkmates Black in one move"]
 [FEN "5nRQ/6PR/1p6/p2P4/p1B5/3Pp3/8/1b1K4 w - - 0 1"]


user25196: This is easy.

 The Black king is somewhere on the 8th rank. It is checkmated in one move by pawn x knight on f8, queening. Now the new queen is giving check, and the rook on h7 prevents the king from going anywhere, resulting in checkmate.


user7297: A solution may be that it used to be possible to

 promote to an enemy piece, as mentioned e.g. here. This loophole has long been fixed in the official rules, but under old rules it was allowed to do 1. g7xf8=(black)K which would be mate.

Would that really explain

 the retrograde-analysis tag?

Perhaps, in the sense that

 we do need to go back in time.


user25189: Looking at the problem as if the black King is currently invisible...

gxf8=Q#
All squares are now covered. So where ever the black King is hiding, the game is over.


15543: <p>Here's a question that's a little off the usual format, but problem questions like this have been done before and are on topic, so here we go!</p>
<p>Question: Can you find the mysterious mate in one? The trick is not board rotation, removing a piece, retracting any moves, completing a move, or an illegal move-there is but one solution. Have fun solving!</p>
<pre><code> [Title &quot;White checkmates Black in one move&quot;]
 [FEN &quot;5nRQ/6PR/1p6/p2P4/p1B5/3Pp3/8/1b1K4 w - - 0 1&quot;]
</code></pre>
 25196: <p>This is easy.</p>
<blockquote class="spoiler">
<p> The Black king is somewhere on the 8th rank. It is checkmated in one move by pawn x knight on f8, queening. Now the new queen is giving check, and the rook on h7 prevents the king from going anywhere, resulting in checkmate.</p>
</blockquote>
 7297: <p>A solution may be that it used to be possible to</p>
<blockquote class="spoiler">
<p> promote to an enemy piece, as mentioned e.g. <a href="https://en.chessbase.com/post/study-of-the-month-2018-07" rel="noreferrer">here</a>. This loophole has long been fixed in the official rules, but under old rules it was allowed to do <code>1. g7xf8=(black)K</code> which would be mate.</p>
</blockquote>
<p>Would that really explain</p>
<blockquote class="spoiler">
<p> the <a href="/questions/tagged/retrograde-analysis" class="post-tag" title="show questions tagged &#39;retrograde-analysis&#39;" rel="tag">retrograde-analysis</a> tag?</p>
</blockquote>
<p>Perhaps, in the sense that</p>
<blockquote class="spoiler">
<p> we <em>do</em> need to go back in time.</p>
</blockquote>
 25189: <p>Looking at the problem as if the black King is currently invisible...</p>
<blockquote class="spoiler">
<p>gxf8=Q#
All squares are now covered. So where ever the black King is hiding, the game is over.</p>
</blockquote>
