From A Night at the Opera, the infamous contract scene:
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 are 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!
(hat tip to Wikipedia)