An example of this happened in my favour this evening. Blinds were 100/200 and I was on the big blind with 23o. 6 of us saw the flop for a pot of 1200. SB and me checked. Player after me bet 800. Everyone folded to me. I'd completely missed so picked up my card protector and moved my hand to my cards ready to fold them. The other guy threw his cards onto the muck at which point I quickly stopped my folding action (my cards were not yet folded - i'm quite entitled to move the cards in my area however I want for some sort of read). I win the 2000 pot because my opponent has mucked his cards.
It's one of those rules that sounds a bit silly (why would you muck a winning hand), but it does happen, so it's worth being aware of
Simple rule of thumb: don't muck your cards until the dealer has awarded you the pot.