I'm not sure what "officially" constitutes a mucked hand. I've seen dealers snatch uncapped cards out from in front of players before. And because cards are often slid into the muck, and not placed neatly on top of the muck, the cards are often unretrievable. I've seen hands where a player meant to "fold and show" what turned out to be the best hand, but because there was no verbal announcement of "fold" and the player folded their hand face up, they won the pot. I saw a hand where Roland DeWolfe showed one card and announced K high. His opponent held on to his cards refusing to show until Roland showed the other card. Instead Roland turned his K back over and attempted to shove both of his cards into the bottom of the muck. The dealer prevented him from mucking his hand and took his cards from him. Only then did his opponent turn over his Q high hand as the dealer turned over De Wolfe's K-5. Even though he had been prevented from mucking his hand because action had been on him and he was required to show, the pot was given to his opponent because he had not shown both of his cards. I might have argued that he had not mucked his hand, only attempted to, but was prevented from doing so by the dealer. If his hand was dead then the dealer shouldn't have turned his cards over. But I'm not a touney official.
I think cdidit and luckyhearts said it best. This could all be avoided by being deliberate in your actions. Don't be fancy! if you mean to fold, say "fold" and fold your cards. If you want to show, just turn them over. Don't be trying to do magic tricks with them. And by all means cap your cards, or be ready to play hot hands with the dealer. I mean you better be lightning fast and rap them hard enough to leave bruises on their knuckles. I usually bring something specifically for capping my cards with me. I know some people just use one of their chips. But what happens in the case that you, like Estelle Denise, don't have a chip to cap your cards with because you have just shove all-in and the dealer snatches your Aces out from in front of you. In a pinch, I will take off my watch and use it to cap my cards with. It's best not to take chances and to be clear in about your intentions.