I take the bus in every day to work. For some reason, some bus drivers don't seem to realize that people occasionally would like to get off the bus, and don't unlock/open the back doors. This gives rise to the fearsome cry of, "Back door!" This is usually yelled by some old Asian person whose command of the English language is limited to those two words.
A stand-up comic had an amusing observation about this phenomenon. He's noticed that tourists, a lot of whom must think of San Francisco as nothing but a roiling hotbed of homosexual revelry, get these horrified looks on their faces when they hear people screaming "back door".