Yellow Cap Switch

The Yellow Cap Switch is an unused object in Super Mario 64. In game there are only three Cap Switches: red, green and blue corresponding to the Wing Cap, Metal Cap and Vanish Cap respectively. In the game's files however, there is a fourth Cap Switch associated with the colour yellow. Yellow boxes are used in game, but normally for miscellaneous rewards such as an amount of coins or a 1-up.

The decompilation of Super Mario 64 reveals that the blocks the Yellow Cap Switch activated was intended to give the player a Koopa Shell.

Since all stages with a Cap Switch are designed around the Cap Switch's powerup, this indicates that an entire stage (and a secret star) was likely cut from Super Mario 64. This stage would likely have to be designed around riding the Koopa Shell as mentioned earlier.