As in the Marx Brothers:
No doubt I'm the 5000th caller with this point, but Krugman lifted the "sanity clause" line from the Marx Bros., or, if you prefer, from George S. Kaufman and Morrie Ryskind who wrote the screenplay for "A Night at the Opera". This is in the famous "contract scene". Google it and you'll get a Youtube hit (speaking as a cog, among the cognoscenti, A Night at the Opera is generally considered the Bros. second-best film, after Duck Soup).
2006-2011 archives for The Daily Dish, featuring Andrew Sullivan