Mystery Goomba as seen from a distance
The Mystery Goomba is a member of a Goomba triangle in Bowser in the Sky that is placed over a pit, causing it to instantly descend to the death barrier upon being loaded. Thus, it exists in a location that cannot be reached, and only for one frame at that. In addition, the radii of the Goomba triangle and the Mystery Goomba do not overlap, so it cannot be interacted with anyway.
While it is possible to clone the Mystery Goomba, for all intents and purposes the coin inside the real thing has been deemed the new Impossible Coin.