The black chocobo glitch occurs in early versions of Final Fantasy V once the player has access to the black chocobo in the Merged World. Landing the black chocobo in the middle of the small grove surrounding the Guardian Tree will make it impossible to retrieve it ever again. This is a harmful bug, since the black chocobo is the only way to get to North Mountain and the Phoenix Tower. The bug happens because the player cannot traverse the grove itself on the world map, as touching either side will enter it.
This bug was fixed in the Advance release by making it impossible to land in the grove.