An Invincishroom is an item that appears in Mario & Luigi: Superstar Saga and its remake,Mario & Luigi: Superstar Saga + Bowser's Minions . They can only be won at the Game Arcade in Little Fungitown.
In the original Superstar Saga, the Invincishroom resembles
a 1-up Mushroom except it's a bit darker green in color and has red spots on it.
The Invincishroom is said to either give the user great powers or to act just like a 1-Up Mushroom. However, when Mario tries one, he turns green and gets a rare disease known as Bean Fever. According to Dr. Toad, Mario gets Bean Fever because he is not used to eating different Mushrooms from different regions. Mario was going to turn into a bean in a few days but was saved after Luigi got some Crabbie Grass for him.
In the remake's Minion Quest, Captain Goomba and the minions were tasked by a Toad in Teehee Valley to find the Invincishroom. But when they do, they instead give the Toad a Poison Mushroom because Captain Goomba thought it looked cooler. It's possible that the same Toad was responsible for Mario's Bean Fever by making the Poison Mushroom a prize in the Game Arcade.