The Liar's Paradox
The Cretans are always liars.
(A Cretan philosopher, of course.)
The trademark version of this classic logic puzzle, along with very possibly the perfect final exam question, has been posted by Frederic Glaize at Le petit Musee des Marques.
A trademark of the word "COUNTERFEIT" -- for clothing and accessories, no less?
Setting aside conundrums of registrability (and yes, there's an English-language U.S. counterpart), imagine a "COUNTERFEIT®" handbag that is allegedly counterfeited by another handbag maker and clearly labeled "COUNTERFEIT." Which is the real counterfeit? Exactly.
Bonus point: What iconic starship captain was a master of using the liar's paradox to attack unfriendly artificial intelligences?