Mystery Goomba

The Mystery Goomba is a specific, oddly placed Goomba that can be seen in the Bowser in the Sky course in Super Mario 64. This Goomba is notable for the precision required to view it, and the impossibility to interact with it in most ways using known tech.

Cloning the Mystery Goomba
If Mario carries a despawning object and the Mystery Goomba loads into the held object's memory slot, Mario [//youtu.be/zy4LiwZ3Uqk can carry a duplicate] of the Goomba. It typically damages Mario once thrown and becomes intangible on contact, like other objects that have been "[//ukikipedia.net/wiki/Cloning cloned]" using this memory glitch.

By grabbing a ledge, the cloned Mystery Goomba can be released without damaging Mario. He can then stomp the Goomba, turning it intangible. The clone does not disappear or drop a coin after being stomped.

Where the Mystery Goomba spawns
Super Mario 64 uses special objects to spawn, or load in, several of the same entity in set formations. Among these is the "Goomba triangle". When Mario is within a Goomba triangle's activation radius, it spawns three Goombas in a lateral triangle around itself.

Upon spawning from a Goomba triangle, Goombas instantly descend to the first floor directly below them. This is likely so that they are grounded when they first spawn, instead of falling to the ground. The specific Goomba triangle in Bowser in the Sky is placed such that only two of the Goombas will spawn above the nearby platform. These two Goombas appear as normal.

However, the remaining Goomba, the one known as the Mystery Goomba, is spawned off to the side of this platform. The first floor directly below it is the death barrier: a special kind of floor that forms the bottomless pits throughout Super Mario 64. Therefore, the Mystery Goomba appears far below the rest of the course.

Why the Mystery Goomba despawns
Like Goomba triangles, each Goomba has its own activation radius. If Mario is within this radius, the Goomba spawns. Otherwise, it despawns, becoming invisible, intangible, and inactive.

For Goombas that are spawned by Goomba triangles, Mario must be within both the activation radius for the Goomba triangle, and the one for the individual Goomba. Otherwise, said Goomba will despawn as described above.

Since the Mystery Goomba is placed so far below the rest of the course, the Goomba triangle activation radius will never overlap with its own activation radius. Mario physically cannot be within both radii at once.

Therefore, the Mystery Goomba despawns on the next frame after it is loaded. (Super Mario 64 runs at a rate of 30 frames per second.) After this, it will never respawn.

All of this means that it is difficult enough to notice the existence of this Goomba in the first place. It is harder yet to view it in-game, and seemingly impossible to interact with it through legitimate means, before it despawns.

Theories
There are at least two possible theories to explain the existence of the Mystery Goomba.

The first is that three Goombas were intended to spawn on the same platform, and the Mystery Goomba's failure to appear there was an oversight. The unusual spawner behavior could be a direct consequence of the level design being changed at some point, [//youtu.be/iPILIf7ru48?t=265 as is speculated for other courses]; a previous version of Bowser in the Sky could have had all Goombas positioned properly.

The second is that the level designers decided to use a Goomba triangle to spawn two Goombas onto the nearby platform. In this case, the fact that the Mystery Goomba exists at all, even for one frame, is technically an oversight.

The more interesting question is whether it is possible to interact with the Mystery Goomba before it disappears. Super Mario 64 experts have attempted to do so, to no avail, in the name of collecting the coin it would drop upon defeat.