1. Make any Hellfire Amulet.
2. Equip only the amulet.
4. Take off the amulet.
5. The passive bonus is now locked onto your character. Regardless if you leave the game.
This is pretty obviously a major problem and we can assume the devs will be hotfixing it as quickly as human possible. Exploiting it would be a bad idea, since at least you’ll get rolled back and at worst suspended or banned.