At the end of the Monty Python sketch "Biggles Dictates a Letter", there's a voiceover (sounds like John Cleese to me) saying: "Next week, part 2: Biggles Flies Undone"
https://getyarn.io/yarn-clip/991e5b96-4e4b-42b4-b45e-801eef914a26
I've always thought this was a pun with two possible meanings: "Biggles's fly is undone", i.e. the fly of his trousers is not properly closed; and "Biggles Flies on Dun" where "Dun" is some location in Europe. But I realized that I've never heard of a place called "Dun".
I searched for "Dun", "Dunn", "Donne", and "Donnes". I found that France has a "Village of Dun" and a "Town of Dun" but both are tiny and don't mention any notable history.
Maybe there is no second meaning; maybe it's just a phrase meant to trick your mind: You think you are hearing "Biggles flies on" and then the last word is revealed to actually be "undone". (Humorist Dave Barry calls this comedy technique "judo": you set up an expectation and then instead of fulfilling the expectation, do something absurd.)
Is this a pun? If so, what is the second meaning?