Poison Mushroom

The Poison Mushroom is a rare anomaly said to manifest in personalized copies of Super Mario 64 if the player is skilled at the game. The Poison Mushroom has been reported to appear in place of regular 1-Ups, or out in the open without being spawned. When a Poison Mushroom is spawned, it will supposedly chase Mario down until it catches up to him. When touching A Poison Mushroom, the player can supposedly lose a life, take damage, or enter a "dizzy" state. This can cause the player's controls to be inverted, or slowly lose health, similarly to the poison gas in Hazy Maze Cave. However, this state usually wears off before the player can be killed by it.

Several have reported being able to find the Poison Mushroom in the files of the game, even if the Poison Mushroom does not spawn. The file name is inconsistent, though several have reported the name to be "PEACH120.Dream". This name has so far gone unexplained, though may be fake due to these claims being unconfirmed.

Theorizing
Many people have said that the maker of the LEGO Mario Poison Mushroom had a somewhat personalized copy of Super Mario 64 as the LEGO variant of the mushroom makes Mario dizzy, similarly to one of the reported effects of the Poison Mushroom. Some have also used this to explain some other oddities in the sets, such as a Thwomp being placed in a lava themed set.