The social law that if you hear a reference to an arcane or forgotten word, idea, or person, you will encounter that word, idea, or person again in the near future.
"Yesterday, I learned the word 'soporific,' and then it was used on the Daily Show tonight! It's Darby's Law!"
"Oh my god. There was a major Darby's Law today. I hadn't heard about Bill Nye the Science Guy in years until we talked about him in English class. Then he was in my science textbook the next period."