Star World

Not to be confused with Star Road, a supposed secret hub world with several unique levels associated with it.

Star World is the name given to a supposed level found in some personalized copies of Super Mario 64. It reportedly takes the appearance of a blue plane decorated with several star shapes, similarly to World Star from New Super Mario Bros. 2. It can appear as a small level, a full course, or even a Cap Switch level. The level can reportedly be accessed through a painting, a secret warp, or by looking upwards in a certain area similarly to Tower of the Wing Cap.

Description
Star World is described as a blue and cyan plane with several star shapes scattered around it as decoration or platforms. A majority of the level is also said to be transparent, though this is not always the case. There are usually several star-shaped platforms used to navigate to higher parts of the level. Notably, no enemies have consistently been reported in this level.

Star World has been associated with the supposed Rainbow Cap Switch, as well as the Rainbow Cap and/or Starman power-ups tied to the Rainbow Cap Switch, likely due to the level's association with stars. Most reports claim that this level is where the Rainbow Cap Switch can be found and activated when both anomalies appear in the copy. However, several other reports claim that the level simply uses the power-ups similarly to Wing Mario Over the Rainbow, instead of being the level where the power-ups are unlocked.

Stars
Few stars have been reported in Star World, despite claims of the level sometimes being a full course. The 2 most common stars are obtained by collecting 8 red coins, and reaching the top of the level with the star platforms. Other stars reported are much more inconsistent and unconfirmed, but include breaking a yellow ! box somewhere in the level containing a star, defeating a boss, and somehow making one of the decorative stars turn into a collectible star.

Theorizing
It is believed that Star World was removed from Super Mario 64 due to the Rainbow Cap Switch and Rainbow Cap and Starman power-ups being removed. Another theory suggests that the level was scrapped due to time constraints, as it appears very simple and potentially unfinished, including the name's simplistic nature. A final theory suggests that the level's theme and design clashed with the other levels and overall theme of the game, causing it to be removed and possibly replaced with another level.