The black chocobo glitch occurs in the early version of Final Fantasy V once the player has access to the black chocobo on the Merged World. Landing the black chocobo to the middle of the remainder of the Great Forest of Moore will make it impossible to retrieve the 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 bug happens because the player cannot traverse the forest itself on the world map, as touching either side will enter the forest.
This bug was fixed on the Advance release by making it impossible to land in the forest.