The Wario Apparition

The Wario Apparition is a well-known anomalous entity in Super Mario 64. They are a large, disembodied head of the Wario character that can be found in the Castle Basement's hallway leading to Dire, Dire Docks and Bowser in the Fire Sea. The Wario Apparition can be spawned by the player's subconscious desires to find Wario in the game.

The Wario Apparition, upon being encountered, will chase the player down the hallway, which will have extended to be seemingly infinite. The Apparition will also chomp at the player and laugh maniacally. Upon being caught by the Wario Apparition, the game will either crash or take the player to the game over screen regardless of lives.

If you believe that the Wario Apparition is manifesting in your copy, it is recommended to power off the console immediately as the Wario Apparition can reportedly manifest into reality due to gaining power from being perceived. The most dangerous effect of the Wario Apparition will only manifest after it has been encountered, particularly after being caught and/or when the victim is attempting to describe the entity. They can suffer memory loss, seizures, and also stroke-like symptoms.

Theorizing
It is believed that the Wario Apparition is a common manifestation of the Anomalous Mental Deterioration anomaly due to it's symptoms, as well as it's generally disturbing nature. It is also believed the anomaly takes this form due to an unused Wario boss fight potentially being scrapped from Super Mario 64.