apple of discord

a thing that is the source of a trouble, particularly by making people argue over it

The idiom "apple of discord" refers to a source of conflict or discord that arises from a seemingly innocent or insignificant issue. The origin of this idiom can be traced back to Greek mythology. According to the myth, Eris, the goddess of discord, threw a golden apple into a gathering of gods and goddesses with a label that read "for the fairest." This act sparked a dispute among the goddesses Hera, Athena, and Aphrodite, leading to the events that eventually led to the Trojan War. This expression is used to describe a specific issue or topic that becomes a significant source of conflict or disagreement among individuals or groups.

That stupid prom dress has been their apple of discord—they've been fighting over who gets to wear it for days now!
This problem seems to be an apple of discord between the Soviet union and the USA.
The decision to implement a new dress code policy became the apple of discord among employees, with some arguing it was necessary for professionalism while others felt it restricted their individuality.
The proposed changes to the school curriculum became the apple of discord among parents, as they held differing opinions on the importance of certain subjects.
The division of household chores became the apple of discord between the roommates, leading to constant arguments and tension in the living arrangement.

(classical mythology) a golden apple that ignited a divine rivalry, leading to events that caused the Trojan War