Apple of Discord

In Greek mythology, the Apple of Discord (Ancient Greek: μῆλον τῆς Ἔριδος) was a golden apple dropped by Eris, the goddess of strife, at the wedding of Peleus and Thetis. It sparked a vanity-fueled dispute among Hera, Athena, and Aphrodite that led to the Judgement of Paris and ultimately the Trojan War.

In common parlance, the "apple of discord" is the core, kernel, or crux of an argument, or a small matter that could lead to a bigger dispute.