Quest ransack

Quest Ransack, as introduced in Update 19, is a stacking penalty for repeatedly playing the same quest. You may play each difficulty level once per character, per incarnation without incurring Quest Ransack penalties. After that, each playthrough will add a 20% stacking penalty to base XP, up to a maximum of -80%. 18 hours after the most-recent completion the penalty is reduced by 50% (and again 18 hours after that). This 18 hour timer restarts each time you run a quest, so if you never leave a full 18 hour break between runs of the quest you will never get any reduction in the 80% penalty, and if you never wait a full 36 hours you will always have some penalty.

You can check your currently penalized quests by typing the /ransack command. If used while inside a quest, this command will also tell you which optional objectives were already completed in the last 18 hours, and their personal penalty.

Casual difficulty
Casual difficulty is exempt from quest ransack penalties: (see talk)
 * Running quests on Casual difficulty does not increase your XP ransack penalty.
 * Experience gain from quests on Casual difficulty is not reduced by your existing ransack penalty.