The black chocobo glitch occurs in Final Fantasy V once the player has access to the black chocobo on the Merged World. If the player takes the black chocobo to where the remainder of the Great Forest of Moore is, and lands in the middle of that forest, the player will be unable to retrieve the black chocobo ever again. This is a harmful bug, since the black chocobo is the only way to get to North Mountain and Phoenix Tower. The reason for this bug happening is due to being unable to walk on the forest itself on the world map, as touching either side of the forest will result in entering the forest.
This bug was fixed on the Advance release of the game. If the player presses the button when above the forest, the black chocobo will not land on it.