From the Super Mario Wiki
An Invincishroom is a mushroom made in Little Fungitown in Mario & Luigi: Superstar Saga. It is given out as a prize for doing well on Star 'Stache Smash. According to the proprietor of the arcade, it is made from Starmen and 1-Up Mushrooms from the Mushroom Kingdom, though these claims are unsubstantiated. When Mario ate one, he contracted Bean Fever, a disease which affects people who are not accustomed to the food of the Beanbean Kingdom. This disease causes the afflicted to become a Bean in three days. Luigi had to travel to Guffawha Ruins alone to find the cure, which is Crabbie Grass. After winning the Invincishroom, Invincishrooms become sold out for the rest of the game.
In Mario & Luigi: Superstar Saga + Bowser's Minions, the Invincishroom is known as an Invincibility Mushroom and it has the appearance of a Poison Mushroom. In the Minion Quest stage Find the Invincibility Mushroom!, the arcade proprietor bribes the Minions to fetch him an Invincibility Mushroom from a dangerous cave. Neither the proprietor or Captain Goomba know what the Invincibility Mushroom looks like, so the minions end up picking a Poison Mushroom instead by accident, revealing that the real reason Mario got Bean Fever was from eating the Poison Mushroom mistaken for the Invincibility Mushroom.
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