Mob Spawner, immobile skeletons in peaceful.

Placing two mob spawners in creative two blocks away, place skeleton spawner egg in one and spider in the other, skeletons will start spawning around the area.
I haven't done much testing other than recreating it on a couple different maps which had the same result every time. Also this was happening before the java update as well so I think we can rule that out.

Screen shots are about a minute or so apart.