The (0, 0) respawning bug was a bug that existed in all official releases of the Doom engine. When a monster is spawned during play rather than at level start, it has no corresponding mapthing, so the mapthing fields inside it are all set to zero. When the code that performs "Nightmare!" skill or -respawn monster respawning considers such a monster, it will be respawned at the point (0, 0) regardless of that point's nature.