I don't know how it happens, but sometimes when skeletons spawn (night, not spawners) and you don't kill them, they don't disappear and are always facing south. You can hit them or throw a heal potion one time and after, nothings happen (I can kill him using my bow sometimes).
It disappear after reloading world or chunks.
I did a test, it happenned 3 times on 80 (only skeletons).