At a poker table, the "muck" is the pile of cards that players have folded as a hand progresses. Everywhere I've played, if you toss your cards face-down and they land in the muck, your hand is dead -- you have folded, given up. Everywhere except one.
Several years ago, I was visiting a friend in Louisville, Kentucky, and went across the Ohio River to play at Caesar's Indiana (now known as the Horseshoe Southern Indiana), a casino on the largest riverboat in North America. In addition to hosting a World Series Of Poker Circuit Event annually, Caesar's was also known as the home of some really good deep-stack hold'em and pot-limit Omaha games the rest of the year, including the night I was there.
When I walked in, there was a long list for some small-stakes games, but there happened to be an open seat in the biggest game in the room, $10-25 no-limit hold'em, so I sat down. The rest of the players were all locals who knew each other. This is common in non-tourist-destination poker rooms (i.e. unlike Vegas, Atlantic City, or other places where people go specifically to play).
St. Louis is one of those towns -- it's rare to have more than one or two out-of-towners at any table -- so I'm used to being on the other side of this equation, but I've played in enough places that I'm not uncomfortable in this scenario. In fact, I prefer it, because locals tend to talk to each other, and poker isn't as much fun without that social aspect. Playing with people who are comfortable and casual, swapping stories or jokes is a key component of poker's popularity. My biggest complaint about playing in the WSOP Main Event in 2012 wasn't that I busted out on Day 2, it's that there was virtually no conversation at the tables.
One of the guys in the Caesars Indiana game recognized me from a trip he'd made to St. Louis, so before long, I was included in the conversation, because poker players love to compare notes about where they've played and where the good games are. The game I was sitting in was a pretty good one, with just enough loose players to keep the pots juiced. I was having fun and winning a little bit.
After a couple of hours, a very big pot developed, involving three players betting a lot of chips. One of them dropped out when the river card was dealt, but the other two ended up in a showdown after one of them made a big bet and the other made an even bigger raise. The first guy called and turned his cards face up. The raiser slapped his cards down on the table face up, too, but the force of his action caused one of them to turn over and fly into the muck.
No one moved or said anything, including the dealer.
The card in question was nearly buried among the discards, but was sticking out about a quarter of an inch. After a moment passed, the raiser said "It's the jack of hearts," then leaned forward, grabbed it by the edge, pulled it out of the muck, and exposed it for all to see. It was indeed the card he had named, which gave him the winning hand.
In a tournament situation, there's no way that would have been permitted. You'll never see it on a televised poker event. Once that card hit the muck, it would be considered as dead as if it had flown off the table.
This wasn't a tournament, but there were several thousand dollars at stake. The other player could have protested, yelled, argued, and made the dealer call over a floor supervisor, who likely would have ruled that his opponent's hand was dead and the pot should be pushed to the player who held the only still-valid hand. Instead, the losing player nodded at the winner, said, "That's good," and folded his hand. Not one person at the table objected.
Why? Because these guys all knew each other. They played in this big game every week. Sure, they were out to take each other's money, but they wanted to do it by out-playing their opponents, rather than winning by a technicality.
In the long run, the good poker players will come out ahead, and the bad players will lose, but a game like that will go on for a long time because of the social factor -- which has nothing to do with the cards, in or out of the muck.