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Its called "Bluffing for a muck", and there is no rule against it.
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Let me refer you to
Robert's Rules of Poker, under Section 3 > The Showdown:
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2. Cards speak (cards read for themselves). The dealer assists in reading hands, but players are responsible for holding onto their cards until the winner is declared. Although verbal declarations as to the contents of a hand are not binding, deliberately miscalling a hand with the intent of causing another player to discard a winning hand is unethical and may result in forfeiture of the pot.
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