Grubba's Gloomy Circus

Grubba's Gloomy Circus is an anomalous level found in some personalized copies of Super Mario 64. The level is described as a dark amusement park-esque area that is very hard to navigate due to poor lighting. Some music from this level has been uploaded to YouTube, which can be listened to here and here.

Description
The main part of Grubba's Gloomy Circus is described as a regular looking circus filled with enemies from all throughout the game. Usually, these enemies are changed cosmetically to appear like clowns or other entertainers seen in amusement parks and circuses. Some have reported that these enemies have sad expressions on their faces, though this is likely exaggerated. There are multiple tents and attractions, though only some are able to be entered. The most commonly reported attraction is a maze made of mirrors, which reportedly leads to this level's sub-area, a basement with a stark change in tone.

The basement is described as a dark maze lit with torches with paintings of Princess Peach on the walls and enemies such as Boos and Scuttlebugs. A notable occurrence that can supposedly manifest in the basement is that staying in the dark too long will slowly damage the player, similarly to drowning in water. The only way to avoid this is either carrying around a unique torch object that can be picked up, or somehow keeping the torches on the wall lit up with something like the Fire Flower. This basement reportedly also has a unique room, most commonly a bedroom, which can contain readable love letters to Princess Peach.

Missions
The first five stars in this level are reportedly found by defeating enemies performing in a tent, navigating the hall of mirrors, and freeing a Chain Chomp that was performing in a tent. There are also reports of a 8 red coins star in the basement maze, and a star for finding a special room in the basement.

The sixth mission is the most interesting one, as it is remarkably consistent between different versions. It begins with the skybox of the main area changing, typically to the skybox seen in one of the Bowser levels. Upon entering the only open tent, it will transform into a boss arena, where the player will fight the titular Grubba, apparently using a similar model to Bowser. His attacks are reportedly throwing his hat like a boomerang, summoning enemies from his hat, spawning a clone of the player to attack him, summoning Unagis that circle the arena, throwing said Unagis, and using a ground pound similar to Bowser's. The arena also reportedly gradually fluctuates in size. Grubba is most commonly damaged by jumping or ground pounding on his head when his head is exposed and none of the enemies he summoned are on the field. Grubba's fight reportedly has several phases where different gimmicks are added, such as hazards being added to the arena and Grubba summoning a clone of Mario which is reported to mimic the player's movement, though these phases aren't always reported. Upon being defeated, some claim that Grubba says to tell Princess Peach he loved her, confirming the supposed obsession that can be observed from the love letters that some report in the secret bedroom area.

Theorizing
This level is notable for the boss character, Grubba, who appeared in Paper Mario: The Thousand Year Door, which released several years after Super Mario 64. This has led to some to suggest that this level is fake, though there are 2 potential explanations for this anomaly. One proposal is that Grubba was originally made for this level, though after it was scrapped they recycled the character for Paper Mario. The other is that this character was made through Coincidental Character Creation, though due to the design being relatively the same this is slightly less likely. Some have theorized that the bedroom sometimes found in the basement is Grubba's, and this is supported by his defeat dialogue, though that isn't present in all reports.