A clue from today's New York Times crossword puzzle: "Mathematician Post or Artin". (Four letters. If you don't know the answer, click on the links.)
The crossword blogs (here, here, here) think this was an unfair clue; this one says that "Neither [...] will be familiar to most solvers, or even to all mathematicians."
I got this with no problem. But it took me a moment, because the son of the Artin the clue was about was one of my professors as an undergrad. A few commenters here and there say they needed some of the crossing letters to decide which Artin the clue referred to.