Skeletons occasionally spawn with no AI and cannot be killed. They still count as monsters, however, and prevent sleeping if they spawn near the player's bed. They don't burn in daylight, and only relogging into the world appears to fix the problem. I have a feeling this is a result of the NoAI data tag reacting poorly with the Skeleton's new AI system, and therefore, they spawn with no AI. Maybe the AI gets its information from the same area as the NoAI data tag? I've only had this occur twice, and it is a random occurrence that can happen in any gamemode with no required help from the player. I have not attempted to remove it using /kill @e[type=Skeleton].