Trickiest Puzzle Ever - Mate in 1

d7 to d5 is an absolute blunder because it allowed mate in one instead of escaping with Kc6-c7 with Black still being down material but still playing.

You are correct. Four decades as a TD/arbiter have shown me that when people do not understand something and have continued to not understand after an explanation then they need a full and complete explanation to compensate for the lack of understanding shorter explanations. I have had to explain tournament rules to IMs and GMs. Many other TDs/arbiters have had to do the same thing. A high rating does not guarantee an understanding of tournament rules (a fantastic understanding of the rules of play, but not necessarily an understanding of the rules for running tournaments).

You're right that was an oversight. My bad

About 11.5 hours instead of 12, however nobody but Arisktotle got it (he recognized the time-worn theme).

White finishes queen-side castling by transferring the Rook from a1 to d1 and delivering checkmate. Yes, this is considered a joke puzzle, though it is sometimes listed as "White to play and mate in one half".

Except that your puzzle is wrong. Black's bishop on f4 prevents the queen side castle as it puts white's king in check, thus an illegal move.

Please take another look.

The Black Bishop on f4 pins the White Rook on e3 to the White King, but does not check the White King (because the White Rook is in the way). Queen side castling (starting with the King on e1 and Rook on a1) is legal and the puzzle diagram is of the middle of the resulting position after moving the King and before moving the Rook.

Ok, i stand corrected, I forgot to put the rook on e3. But then again, who the "ef" (Pronunciation for the French translation of the word "seal") annotates a board's position with only half a castle completed? At least in the original post, you could eventually deduce that Black's last move was d7-d5.

The only reason to deduce that Black's last move was d7-d5 was because that blunder (Kc6-c7 was much better) was the only move that made the win possible. In the "mate in half a move" puzzle deducing that the diagram was from half way through castling was the only move that made the win possible. And yes, it is a joke. I am not sure, but it might have first been widely published in a column by Sam Loyd, a general puzzle maker that was well known for using tricks (many involved math and geometry).

I didn't know about Sam Loyd, interesting character, I looked him up. Interesting character, admittingly an outstanding chess player, but also with a reputation for being reprobate and a snake oil salesman. Nonetheless, here's the annotation for white last move at the beginning of that puzzle... o-c.

Like the others this is basically a joke puzzle. The missing information remains hidden until the puzzle is solved.. The last thing anyone would do is give away the plot by specifying "o-c" with the puzzle. The fun of joke puzzles is not to deduce the solution but to "guess" which nearby rule change would lead to an elegant solution!

Note: there is a composition type (MDR) which exactly defines which incomplete or illegal moves are permitted preceding a diagram. However, you must first prove that an illegal or incomplete move was played and then correct or complete it per the FIDE laws as if you had played that move in a chess game and pushed the clock! In jetoba's diagram you cannot prove anything and you cannot solve it by completing the move per MDR!

En passant.

This puzzle wasn't very tricky, found it in 5 seconds.