My opponent resigned in a winning position!!!

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Avatar of larrysupercpu
In that position, I played Qf8, thinking that I can back-rank checkmate, and my opponent resigned. But when I went to analysis, I’m shocked——that move was a blunder! Why? Because if he blocks with the rook, it will make a discovery check! Thank God he didn’t see it…
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https://www.chess.com/game/live/82903985439