Something like that happened to me a few weeks ago at the Taj. I was playing 1-2 NL when the guy in the 10 seat went all in. Being the only other person in the hand, I said "Call" and flipped my cards over. He mucked his cards, even though he had me beat. He said that he thought he had heard me say "Fold," although everyone else at the table heard me clearly. The floor was called, and they obviously gave me the pot, which I split with him (because I'm generous and gullible.... He whined that he had no more money and I felt bad for the poor sap, until he pulled out another buy-in 20 minutes later... jerk...) Anyway, sum of the story is that, yes, when you muck your hand, it's dead. And I've found that's pretty universal.