The Yellow Cap Switch is an unused object in Super Mario 64. There are only three Switches encountered in gameplay: 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 Switch, which is yellow. A transparent variant of the Yellow Box is associated with it in the files. According to the decompilation of Super Mario 64, the blocks the Yellow Cap Switch activated were intended to give the player a Koopa Shell. This usage of the Yellow Boxes was kept in the final game for a few of them. It is speculated that the shell originally used by the Yellow Cap Switch was to be yellow colored, so that it matched the switch color.
It is theorized that initially, there were four Switch stages with the Yellow Switch being cut at some point in development. These would have corresponded with the four switches found in Super Mario World's Switch Palaces. The fourth Cap Switch stage would presumably have awarded the player another Power Star through a Red Coin Secret Star. Obtaining this star would probably have involved the usage of the Koopa Shell as the Yellow Boxes, which were originally activated by the Yellow Switch gave the player a Koopa Shell.
It is not known why the Yellow Switch was cut from the game – however, it may have involved the level design allowing very limited use of it; in the final game, the Koopa Shell is hardly used. It could have also been cut because Koopas may have made it redundant as they drop rideable shells anyway.
It is also unknown if before the Yellow Switch was removed, the Stars and 1ups inside the final game's Yellow Boxes were still in Yellow Boxes, a different type of box or another situation altogether.
Given that two of the cap switches are accessed from the Castle as secret levels (namely, the Wing Cap and the Vanish Cap) and that only one cap switch can be found in a course (Hazy Maze Cave, the Metal Cap) the Yellow Cap Switch's level could have been accessible through a course world to balance the amount of cap switches found in the castle vs cap switches found in courses.
- If the Yellow Switch is retrieved from the game's files it is already pressed when it spawns.
- This may be because the Yellow Blocks are always solid in the final game.
- The Yellow Boxes are the only boxes in the game to have an exclamation mark (!) instead of a question mark (?) on them.
- It is unknown whether this was changed when the Switch was removed or if this was always the case to differentiate it from the Cap Boxes.
- Some Yellow Block stars, such as on BOB's floating island and beside the climbable points in JRB, are only contained in Yellow Blocks in the international versions. In the Japanese versions (including, bizarrely, the Rumble Pak update) those stars simply float freely.