Teams who reached the third borough of 2012, Brooklyn, were greeted with this clue at Brooklyn Brewery: Clue #4 for emailing out after #2 is complete
I honestly expected this to be one of the easiest clues, but my impression is that most teams found it to be the most time-consuming of the clues, if not necessarily the most vexing. For while it was very clear what to do with it, actually completing the puzzle was another matter.
Either way, I think it captures the essence of what I strive to create in each clue. It requires some amount of knowledge or skill, can’t be solved with a simple internet search, will take a while to figure out, and it is not perfectly clear what you do with it until after you solve it (and, sometimes, not even perfectly clear then).