121st Star

The 121st star is a common rumor in Super Mario 64. There are 120 total stars to collect in the game, though some rumors state there is a hidden star. There are several rumors on how to get it. However, if the Personalization A.I. adds a new course, this can be collected easily, and nobody seems to realize it. It is currently unknown if this is a cognitohazard similar to the Shared Nightmares or just oversight.

Unused Level 99
Unused Level 99 is an unused level the files of Super Mario 64. There is a star in this segment, and has been dubbed a possible 121st star. However, the level cannot be added via ROM Hacks to obtain a 121st star due to every area having a star, and Unused Level 99 have no teleporters to other levels.

Toad's Tool 64 Star
Via using the emulator known as Toad's Tool 64, a collection of strange objects can be found in Cool, Cool Mountain's slide section, dubbed the Toad's Tool 64 Star. A notable object in this cluster is a star. However, it does not function as a star due to being a side-effect of Toad's Tool requiring an extended ROM, which can cause a multitude of glitches, including creating these objects.

Princess' Secret Slide 100 Coins
The most common of the rumors is that you can collect 100 coins in The Princess' Secret Slide and get a 100 coin star. This is impossible in regular copies without glitches.

Stained Glass Slide
There is a supposed slide level called the Stained Glass Slide found by using the 120-star cannon in the Castle Grounds to shoot into the stained glass, with the 121st star at the end.

Feeding Tuxie
The anomaly known as Feeding Tuxie has sometimes unlocked a 121st star.

Mario's House
A minor anomalous area known as Mario's House contains a 121st star in a hidden room.

Behind The Waterfall
A supposed hidden level known as Behind The Waterfall is rumored to have a 121st star.

Behind The Portal
There is a rumored legitimate way to get behind the Dire, Dire Docks portal after it moves the first time, and it would yield a 121st star.

Behind The Mirror
There is a supposed way to get behind the mirror in the Mirror Room that would contain a 121st star in a similar fashion to a secret star in Super Mario 64 DS.

Yoshi's Gift
In some copies of Super Mario 64, the 121st star is given by Yoshi when the player reaches the roof of the castle.