For several months now I've been trying to recall the legal term of art for "legal conventional wisdom," or the general understanding that something is the law, even though it was never codified and has never developed as common law. The phrase I was looking for was rule of recognition. Yay, me!
For almost as many months, Mithras has been trying to remember the term for the deliberate errors that mapmakers introduce into their maps to catch copyright violators. The term is trap street. Yay, Google and Wikipedia!