From the Super Mario Wiki
If Captain Toad or Toadette loses a level often enough, a sprout will appear with an Invincibility Mushroom. Plucking it and collecting the mushroom will cause the Toad to glow, similar to a Super Star. However, the effect lasts until the end of the level, making it easier for them to deal with tough enemies, leaving any puzzles within the stage as the only remaining challenge(s), similar to the Invincibility Leaf.
An Invincibility Mushroom also appears in the Minion Quest mode of Mario & Luigi: Superstar Saga + Bowser's Minions. The Little Fungitown arcade proprietor bribes the Minions to fetch one from a cave in the mission Find the Invincibility Mushroom! However, since neither he nor Captain Goomba knows what an Invincibility Mushroom looks like, the latter ends up picking a Poison Mushroom instead accidentally.
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