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.)