Every 7 days, you can obtain a Mystery Quest. This quest can be any rarity, and is given to the Questie you comment with. If the Questie already has 8 incomplete quests, you will be informed of this fact and can comment again once a slot is cleared. If the quest rolled is already on the Questie's profile as incomplete, the Mystery Quest will be re-rolled.
The Mystery Quests include quests you can't get any other way!