While Monty Python became famous for these memorable lines from their "Dead Parrot Sketch", the joke itself predates the British comedy routine... by nearly 1,600 years!
A recently republished collection of some of the world's oldest jokes, Philogelos: The Laugh Addict, contains an ancient Greek witticism written in 4 AD about a man returning a recently-purchased dead slave to his seller. "By the gods, when he was with me, he never did any such thing!" replies the seller.
Fortunately for John Cleese and the rest of his comedic comrades, there were no copyright infringements made on the joke since it was written without prior knowledge of the Greek farce. Besides, the author is long since dead... and unavailable for refund, as well.
If you've never watched the Monty Python sketch, or care to revisit it, here's a clip from a live performance in 1976:
You can also get your own plush "sleeping" parrot in remembrance of the classic British comedy skit.