The 121st Star

How to get it
The 121st star is a mystery star that can be found in Cool, Cool Mountain in certain personalized copies. This star can appear if you can beat the penguin in less than 10 seconds while passing all the checkpoints, you will be awarded a star by the title of "Beating the penguin at his own game". Once you grab the star you'll be teleported back to the hub.

Effects & Guide
Most of the castle will be covered in ice and the paintings will be inaccessible apart from Cool, Cool Mountain, Snowman's Land and a new world called Cold, Cold Crevasse. Toads will be replaced with a glitchy mess of a penguin using Toad's textures. If you attempt to talk to the one on the ground floor he will say "Oh, hey Mario! Thank goodness you're back! A penguin came in the castle and screamed your name. Help us Mario!" Afterwards, the game's music will slow down and corrupt. If you pause you'll hear a fast version of Cool, Cool Mountain's song and the words "I'M NOT HAVING FUN" (Similar to the Textureless Mario Anomaly) with a penguin in the background. Most the rooms are cold and barren with very little colors apart from blue and white. If you go back to the Cool, Cool Mountain slide, the game will crash. If you go to Snowman's Land, the Giant Snowman will become a penguin. Atempting to load Cold, Cold Crevasse will crash your game. AVOID PENGUINS AT ALL COST. Penguins will usually become hostile enemies, similar to Goombas (if you kill 100 penguins, the game will begin to break more on that later). In order to progress you need to have 8 stars and go to Bowser tn the Dark World. The world will become similar to Bowser in the Fire Sea but a cold empty world. All the enemies will be replaced by penguins. After going into the loading zone, you will hear a sped up version of Bowser's laugh. The penguin you race will say "Mario, it's good to see you. I might not be the fastest racer but I will make sure your death is as fast and painful as possible". You will hear the final boss music and Mario will clench his fists. The fight will be very similar to a Bowser fight. if you are a minor then restart your console. If Mario dies he will burst into blood and the penguin will eat his body. if you win the penguin will say"V2hhdCE/ISBCdXQgaW0gYSB3b3JsZCBjaGFtcGlvbi4gQXVyZ2ggaGUgc2FpZCBJIGNvdWxkIGRvIGl0LiBDdXJzZSB5b3UgTWFyaW8hISEgV2FpdCBJIGNhbiBzdGlsbCBkbyB0aGlz" which is likely a result of the Personalisation A.I. being angry that the player beated the A.I. It will then crash the game. if you try to load the save file, a message will appear saying "Up for round two?" and you will have to kill the penguin again.

Theories
Due to the star being commongly seen in a map loader called Toad's Tool, most people think that it is the Toad's Tool glitching due to extending the boundaries of the slide for custom course making. The develoler of Toad's Tool most likely had a personalized copy of Super Mario 64 with this strange anomaly. Others suggest that this is from a scrapped boss that was planned but removed for unknown reasons.