Having unprotected cards mucked is a dealer error, but its one that is also player error.

My favorite dealer error (because it was so bizarre that I had never seen or heard of this one before) was a dealer at Palace Station who was obviously new. I was on the button and tossed my cards in to muck them. For some unknown reason this dealer picks up the cards I mucked and places them on the top of the deck; then he burns and turns (one of my cards became the burn and the other one was the first card on the flop.) The floorman was not happy when he had to come and sort that one out.