For this most recent NBA season, ESPN.com has had in-game win probabilities:
Say a team gets a big lead and has a 90% chance of winning. What is the probability that at some point during the rest of the game, the other team stages a comeback to the point where both teams have a 50% chance of winning?
While neither assumption holds in reality, assume for this puzzle that win probabilities are perfectly accurate and continuous (meaning the probabilities don't make a sudden jump, but smoothly transition from one value to another). Surprisingly, I don't believe you need to know anything about the teams, how much time is left, or really anything about basketball, to solve this puzzle.