I scared the hell out of my players with a gelatinous cube ghost. Completly harmless (they didn't know of course), and unlike live gelationous cube - visible, colored like a ghost of course. It tried to engulf them, without any effect, because it didn't have a body any more. It than followed them, still trying to act as if it still were alive. It eventually followed them out of dungeon and to the nearby village.
Villagers were not happy. It scared the farm animals, annoyed people...
Finally, they got an exorcist to make it go away. Funny times.