Weird Mushroom (item)
From the Super Mario Wiki
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The Weird Mushroom (originally called the Luigi Mushroom) is a power-up in Super Mario Maker. Upon use, Mario will turn taller and slimmer and gain the abilities of Luigi, with higher jumps but lower traction. According to Takashi Tezuka, the form came from a bug that happened in development that made Super Mario skinny. However, Tezuka and the team liked Weird Mario's design, so they implemented him into the final game.
It looks similar to a normal Super Mushroom, but with a short cap and a very thin base. A Super Mushroom can't override the effect of this mushroom, but the Weird Mushroom can override a Super Mushroom's effect. However, a Fire Flower, Mystery Mushroom, or Big Mushroom will override the Weird Mushroom's effects, while Fire Mario, Costume Mario, and Big Mario are unaffected by Weird Mushrooms.
The Weird Mushroom has a 1 in 10 chance of spawning wherever a Super Mushroom is placed. However, it can also be unlocked by clearing all four of the Nintendo World Championships courses. Upon unlocking the Weird Mushroom, the player can use it by shaking a Super Mushroom in the editor.
The Weird Mushroom is exclusive to the Super Mario Bros. game style, however, so if the player changes the game style, every Weird Mushroom object will turn back into a Super Mushroom.