Reverse Mushroom
A Reverse Mushroom is one of the rarest types of Mushrooms in the Mushroom Kingdom. It is the opposite of a Super Mushroom, causing someone who is enlarged to shrink and vice versa. This mushroom can be used against foes.
In Super Mario Bros. 3 Battle Mode version of Mario Bros., they would appear and could shrink/enlarge, or reverse the two players positions on the screen.
This rare mushroom also appears in Mario Party 3. The player who has it can make any player on the board (including himself/herself) go in reverse when they are moving across it, until they reach a fork or area with multiple paths, then the person inflicted can choose to go forward in another direction, or still go backwards. It is Donkey Kong's favorite item.
Strangely, it is one of the only mushrooms to only appear in one Mario Party game.