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, 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.

Example
Day one: Day two:
 * 1) Elite, first time completion: full exp (0% ransack after run)
 * 2) Hard, first time completion: full exp (0% ransack after run)
 * 3) Hard: full exp (20% ransack after run)
 * 4) Hard: -20% quest ransack penalty (40% ransack after run)
 * 5) Hard: -40% quest ransack penalty (60% ransack after run)
 * 1) Hard: -10% quest ransack penalty (30% ransack after run)
 * 2) Normal, first time completion: full exp (30% ransack after run)

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.