I remember reading / hearing recently that two boxed cards, found after the deal, can cause a misdeal.
The reasoning was strange: It would indicate that a player's hand was mucked face up, thereby giving an observant player the advantage of knowing that those two cards are out of play until discovered face up.
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About 4 years ago at a local casino, I was playing at a $1/$2 game where the hand played out to the river, the dealer spread the stub, put the cut card on top, there was a showdown, the pot was awarded, the rake and bad beat was dropped, and the cards were starting to get gathered when someone noticed that the card on top of the stub was not the cut card, but a card from the other deck. "Floor!"
Floor comes over, and sees the issue, and sees a green light on the shuffler. He takes the shuffled deck out of the shuffler, and sure enough, there's the cut card, shuffled in. He starts counting the deck. I realize he's just doing that to give himself time to think. He finished counting and declares "Misdeal" and has the dealer unravel the hand to return chips to everyone. I immediately started to complain that that shouldn't be the correct ruling. However, I didn't even call the flop, so I didn't argue too much. Also, the pot was only $23, so nobody felt like arguing.
When I got home, I found two RRoP rules to support my belief that it's not a misdeal.
But now, upon thinking about the reasoning behind the two boxed card rule, maybe a misdeal was correct. After all, anyone who caught a glimpse of the face of the bottom card could have been erroneously using that info during the hand. It's why the cut card is there, after all...