Here 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 "therein lies the rub" and "that's the rub." All I need to do to get my car back is pay the fine, but here's the rub: my wallet is locked in the glove compartment.

There is the biggest problem or difficulty (with the situation being discussed). Seen in many different variations, including "here's the rub," "that's the rub," and "therein lies the rub," the phrase was famously used in Shakespeare's Hamlet. A: "All you need to do to get your car back is pay the fine." B: "But there's the rub—my wallet is in my car."