Punchline to the contract-signing scene in the Marx Brothers' A Night at the Opera (1935). Groucho is trying to get Chico to sign a contract for an appearance of an opera singer he represents. They agree to disagree on practically every clause in the contract, ripping them out, until only the sanity clause remains.
Chico: Hey, wait, wait. What does this say here? This thing here.
Groucho: Oh, that? Oh, that's the usual clause. That's in every contract. That just says uh, it says uh, "If any of the parties participating in this contract is shown not to be in their right mind, the entire agreement is automatically nullified."
Chico: Well, I don't know...
Groucho: It's all right, that's, that's in every contract. That's, that's what they call a 'sanity clause'.
Chico: Ha ha ha ha ha! You can't fool me! There ain't no Sanity Clause!