Where is the 121st Power Star in Super Mario 64?
There are actually 121 Stars in Super Mario 64, you may just have not noticed it. Officially on paper, there are 120 Power Stars in Super Mario 64. 105 in the standard courses and the 15 Secret Castle Power Stars.
There has long been speculation of a 121st Power Star, but if you played through the game and paid careful attention to the details, you will realize that it was actually always there!
In the third and final Bowser battle - Bowser in the Sky, after defeating Bowser, a giant Power Star appears. Presumably this would be Star #121, but it doesn't count towards the game totals. It just gives Mario the Wing Cap for his victory lap for successfully rescuing Princess Peach and restoring the power of the stars to her castle.
The 121st Power Star - too bad it doesn't count!
Since this star can never be "collected", it will always appear in the golden form when you defeat the final boss. In successive games like Super Mario Galaxy, this would be analogous to a "Grand Star", which is in fact collectible and counts as a star in the game totals.