i think the whole time this show has been going on the "dead people" appearing on the island were thought to be jacob, but i really think that it's Jacob's enemie who is appearing as them. watch from the begining and watch the scene in the season 5 finale where the crew thats carrying lockes body stops at the cabin and the black line around the cabin is broken which scares the ones that work for jacob

But if that's the case, than how come someone was inside the cabin when Ben and Locke went inside in "The Man behind the Curtain" in Season 3? He seemed stuck in there, asking Locke for "Help me".