That is the biggest problem or difficulty (with the situation being discussed). The phrase was famously used in Shakespeare's Hamlet (as "there's the rub") and is now seen in many different variations, including "here's the rub" and "that's the rub." A: "All you need to do to get your car back is pay the fine." B: "But therein lies the rub: my wallet is in my car." Even if they gave me immunity for testifying against him, I knew I could be the target of his retaliation, and there lay the rub.