I've just been looking at some Hidden Abilities to see what might be good to aim for.
One such Ability is Pickpocket. According to the information I've found, it will steal the opponent's held item if the opponent makes contact.
I've noticed that simultaneous abilities generally work in an order that benefits the user. For instance, when an opponent uses Drain Punch on my Ferrothorn+Rocky Helmet, it will absorb HP before taking damage from Iron Barbs and the item, while in theory in a realistic situation the attacker would be hurt when the punch lands, before any health is drained.
So what happens if a Pickpocket with no item takes an attack that would KO it in one hit, but the attacker made contact and is holding a Focus Sash? Would the Pickpocket ability activate first and steal the item, thus allowing the thief to survive, or would the damage be too much for the ability to activate?