The Pit

Not to be confused with Pit of Peril, an anomalous underground course set inside of a large pit.

The Pit is an anomalous are found in some personalized copies of Super Mario 64. It is described as a grassy plains area with a large pit in the center, being the main landmark of the area. The Pit is considered very mysterious as no reports clearly state what is at the bottom of the titular pit, though there have been several theories about this supposed location. The Pit can supposedly be accessed by jumping over a wall in the Castle Grounds, Bob-omb Battlefield, or any other similar grassy area.

Description
The Pit is described as a large flat grassy plane with a large, gaping hole in the center of the level, giving it it's name. Some have reported the unused Dancing Flowers appearing in this area, though these reports are unconfirmed. The only other notable thing about this area is that there is a sign in front of the pit that welcomes the player to the area and talks about The Pit.

Supposedly, whenever a player jumps into the pit, the game crashes. This has caused several theories that either the area is unfinished, doesn't exist, or the Personalization A.I. is trying to prevent the player from entering it for whatever reason. Those who have claimed to have entered the pit do not clearly state what was there, though several have stated something about being unable to escape the area.

Report 1
This video by Youtuber First Name Last Name was the first video documentation of The Pit. It begins with the player, notably playing as Luigi, jumping at the edge of part of the Castle Grounds before being warped to The Pit. They look around, before reading the sign in front of the pit in the level that says:

"Welcome to the pit! If you jump in, you can either have a wish come true, or pay a deadly price."

The player runs around before jumping into the pit, causing static to appear on the screen as the video ends. The description states the video crashed when the static appeared.