Weird Mushroom (item)
From the Super Mario Wiki
The Weird Mushroom is a power-up in Super Mario Maker and Super Mario Maker for Nintendo 3DS. Upon use, Mario turns taller and slimmer and gains 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 can 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 100 chance of spawning wherever a Super Mushroom is placed, except during upload check, where it was changed to never randomly spawn after an update. 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. In Super Mario Maker for Nintendo 3DS, however, the player starts the game with the Weird Mushroom in their item selection.
The Weird Mushroom is exclusive to the Super Mario Bros. game style, so if the player changes the game style, every Weird Mushroom object turns back into a Super Mushroom.
As of the version 1.20 update of Super Mario Maker, if the player taps on a Warp Door enough times in build mode, the door will briefly open to reveal Weird Mario in a variety of different poses and costumes, most of which are based on power-ups or mechanics from the original games that are otherwise absent in Super Mario Maker, such as Ice Mario or Balloon Mario.
Although the Weird Mushroom does not appear in Super Mario Maker 2, Weird Mario will still "answer" if a Warp Door is tapped enough times in a row.