Found this in Genius at Play (a book about the mathematician John Conway):

Conway was at a pig-roast, at which it happened that there was a table of French speakers, and another of English speakers (including Conway). A French person gestures to two spare chairs at the latter table, and asks, "May we?"

CONWAY: Mais oui!

It is his proudest pun, of which he's said, "I've been trying to set it up again ever since."