After killing the ender dragon in 1.8.8, I opened my world in 15w36d in a backup and respawnned the ender dragon with the clay creeper face. When i opened the world again in 15w37a and built the same creeper face, it didn't spawn the dragon and no obsidian pillars were made. I attached the level.dat of my world because the zipped version of the entire world save was 83MB and exceeded the maximum attachment size.