Once upon a time, on Prickle-Prickle, in the season of Bureaucracy, Our Lady took a golden apple as her weapon of choice. When she wasn't invited to a party, she decided it would be more fun to jam the party instead. So she wrote "For the fairest of them all" (watching too many Disney movies, the Pope suspects) on a slip of paper, attached it to a golden apple, and rolled it in.
The golden apple went off like a flash-bang, setting the three goddesses in attendance arguing and fighting over which of them was the intended recipient.
The golden apple is a symbol of Eris, and of screwing with people simply by offering them the chance to be themselves. There is also some speculation that the golden apple is a symbol of a world so grotesque, so badly engineered, and so generally Disney-esque, that hunks of inedible metal hang dangerously from its trees.