Secret

Secrets are collectibles found in some levels of Super Mario 64. They have no appearance, and instead are signified by a yellow sparkly effect. They are found in sets of 5, and reward a star when collected. They appear in Wet-Dry World, Shifting Sand Land, and Tiny-Huge Island. In Super Mario 64 DS, they are replaced with Silver Stars, and do not make any appearances.

Red Coin Replacement
In some personalized copies of Super Mario 64, red coin stars in any level with them can reportedly have the red coins replaced with secrets. The vice versa has also been reported. This is believed to occur when the player is either performing well or poorly, in order to change the difficulty to suit the player's needs.

Anomalous Visibility
Some personalized copies reportedly have secrets appear visible. They usually appear as Silver Stars, though can take several other appearances, such as a special type of coin. Similarly to the Red Coin Replacement, it is believed this occurs in order to change the difficulty for the player.