I've never had the urge to display bumper stickers on my car. But last night it occurred to me that a bumper sticker simply reading ‘THE BUTLER DID IT’ would have the amusement value of being able to claim my car had a spoiler on it :-)
I think I may have missed that one in my list of spoilers everyone knows. Admittedly, almost no-one knows which books the spoiler is *for*! :)
(IIRC there was a straight dope article on it, and there was one famous book which it probably applies to more than others, though it has been the butler in a number of books. I think the tipping point is when servants in the story are sufficiently background that it's a twist that one of them did something, but not so tired that introducing a new suspect is just a cheat.)