Haaa! :)
The first end game study, made to clarify rules concerning check and duple-check, contained a flaw! Well, it did serve it purpose and it did clarify the point.
This reminds me of the play testing we did. Because we placed “testing the game” as more important than “ruthlessly beating our opponent” we allowed liberal amounts of take-back-zies. It was not infrequent that we would make a move and then moments later say “Whoops, your piece can move like that, my move was dumb” and we would take the move back. Sometimes we would backtrack 3 or 4 moves because the error someone made was so glaring. We could easily do this recording the game in reversible long algebraic notation.
Wow! Duple-check and mate and 5 moves. We have yet to have a duple-check and mate in an actual game.