Small Mario

Small Mario is Mario's base form in the 2D Super Mario series. As small, he dies when hit and cannot break brick blocks. Super Mario 64 replaced the powerup system with a health bar and Mario's big form "Super Mario" became the default. All further mainline 3D games would follow course.

Super Mario 64 Manifestation
While absent from the final version of Super Mario 64, Mario's small form has been theorized to have been present in very early versions of the game, due to both it multiple 2D powerups being implemented by the AI. This may mean that Super Mario 64 originally was more directly based on Super Mario World, which is supported by Miyamoto admitting in an interview first getting the idea of a 3D Mario platformer while working on the SNES's Star Fox.

The exact details of the anomaly differ between copies. Some players have reported Mario changing forms at 4 health, while in another copy Small Mario was the default form and Mushrooms functioned as a power-up that gave Mario another whole health meter. In one extreme case, the health system was removed entirely and mushrooms replaced coins entirely in terms of healing, althrough somewhat rarer. These different manifestations could represent different stages in development.

Research into this anomaly is hampered both by it's relative rarity and it's instability. Being removed so early in the game, many copies lack important features such as a model or animations. How the AI handles this varies by copy, with effects ranging from crashes to placeholder models.

Trivia
It is possible that the Life Mushroom in Super Mario Galaxy was inspired by Big Mario's doubled health bar.