I stumbled upon a spider spawner while caving. After lighting the room up, skeletons start spawning inside the room, inside blocks and floating in the air. They were all facing south and did not attack or react. They could not be pushed but could push my character. They could be hit one time with melee and no more times after that. They kept spawing until I destroyed the spawner. After restarting the world, the skeletons were gone and no more spawned.