Rainy Sky Spires

Rainy Sky Spires is an unused and unfinished course found in certain copies of Super Mario 64. It takes place in a series of floating structures in the sky during a rainy night and is usually reported to be accessible in the First Floor, more specifically in an area called The World of Dreams. People with this level in their copy claim it looks like it was scrapped early in the game’s development, lacking objects and having some glitchy walls and textures. Notably, it uses a rain effect that goes unused in the final game.

Geography
The level mostly consists of large stone structures with some additional floating platforms and clouds. The main area has four towers connecting to each other with bridges, with another large circular tower in the middle. Each of the four towers has an interior and windows, some of which seem to have been broken and can be used as entrances or exits. Other notable structures include a castle which can be accessed by a slide off of one of the towers, and a flying ship with a thin layer of accumulated rainwater between two of the four towers, as well as various clouds and other floating platforms.

Speculation
Due to the design similarities to Tower of the Wing Cap, some believe that Rainy Sky Spires is what that stage was originally meant to be, but it was too large and had to be transformed into a smaller stage to save on memory. This is supported by many players who encounter this stage claiming it to have severe performance problems, possibly implying that loading the level requires space the game no longer has access to.

Other Phenomena
Inside the castle, which is sometimes referred to as the "Wind Palace" by the mission select screen, the player can wall jump towards a balcony which leads to a Wing Cap box. It can then be used to traverse the clouds on the edge of the level which lead to the top of one the towers. The player can however decide to instead fly towards the bottom part of the opposing tower and enter an otherwise inaccessible room which contains a single toad and an oven. Reports on the Toad's dialogue vary, with most claiming it to contain only a four dialogue boxes long sequence consisting of the number "9", Other reports stated the Toad's dialogue to consist of incredibly cryptic, often grammatically incorrect references to various philosophical concepts, mostly revolving around the afterlife and certain dynamics between the microcosm and macrocosm. Some players have stated the floating ship to move around the level sometimes, making the mentioned secret room accessible without the use of the Wing Cap.

Another possible quirk is that the level has been reported to have either six or just one star, however none of the reports suggested a 100-coin star to be obtainable in it.