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Tournament Directors Association:
Rule 35: A player who exposes his cards during the play may incur a penalty but will not have his hand killed.
for those who are google impaired
I don't see a rule that specific in Robert's rules of poker, but if you look under the section heading: "Dead Hands," exposing your cards is not listed as a reason to kill a hand. Also under "Showdown," Rule 6 "show one show all" its strongly implied (indeed I don't see any way to read the rule otherwise) that an exposed hand is not a dead hand.
Linky, cause I know google is so hard to use
--Zetack
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One note- this generally is limited to heads-up play, where exposing the hand cannot affect future betting sequences, I believe?