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If someone other than the bettor requests to see and you've mucked the winner, your hand is dead and the bettor gets the pot.

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The variation: only those involved in the showdown may request to see a called hand.

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The rule is in place to help prevent collusion. In the first case described, no collusion is stopped on the given hand, only highlighted (possibly). In the second case, it is impossible to detect collusion. It is rude to repeatedly ask to see the hands of your opponents, and it slows down the game.