when spawning the ender dragon, if you leave the game/unload the chunks/leave the server/stop the server the summon will seem to freeze, on our server it happened at just the right moment to where there was one ender crystal stuck in the summon, and it could not be destroyed, however, the end dragon was still there, and he was indestructable because of the indestructable ender crystal.
It would light on fire from a flame bow but not die, you couldn't even kill it in creative, the beams both pointed up into the sky and at the ender dragon