The Mystery is an item that has only appeared in the Paper Mario Series.
 Game History
It would first appear in Paper Mario as the Mystery? item. If it is used in battle, a roulette will start and will stop on one of these items:
It could also be used to create other items with Tayce T..
It would return in Paper Mario: The Thousand-Year Door. It could be sold for three coins at Niff T.'s shop and Pungent's Shop. When it is used in battle, it can turn into any item.
The Mystery item would return in Super Paper Mario as the Mystery Box. When it is cooked, it will become a random item, though it is likely that it will be a Mistake. It can be bought at the Flipside Arcade for five Flipside Tokens, Downtown of Crag for three coins, or by defeating a Lakitu.
The Mystery Item will be appearing in the upcoming Paper Mario 3DS game.