Pandolfini's Puzzler #4 - Less is More

The fourth article from ProfessorPando has a big trick in it. Take your time and see if you can spot the trap - don't rush in to this one!
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“Lucian,” Professor Pandolfini intoned, “please look at the next setup.”
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“Let’s assume the following. You have White. It is your move. And you have a winning game. 
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Question: “Do you see how to checkmate Black in two moves?” 
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At first, Lucian thought he had the right answer. Then he stopped himself, thought some more, and changed his mind. “I thought I had the right answer,” he said. “But I was wrong. It’s not a mistake, however, unless it’s played on the board.”
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What was the wrong move Lucian thought of first? 
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What was the right move Lucian thought of second?
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Answers Below - Try to solve ProfessorPando's Puzzle first!
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Answer to Puzzler #4
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The first move that Lucian thought of was 1. f8=Q??, which gives stalemate. 
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The second move Lucian thought of was 1. f8=R! (next diagram), which leads to mate next move.
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Take note
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Most of the time one promotes a pawn to a new queen. But sometimes it’s better to go for an underpromotion -- for a knight, bishop, or rook. 
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You might underpromote to avoid stalemate.
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Or you might underpromote specifically to a knight -- either to give immediate mate or a winning fork.
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