This has come up a few times over the course of a couple AP's in our group.
Scenario is that a ghost possesses a party member, let's say the rogue.
The cleric realizes what's going on and attempts to channel positive energy to damage undead.
If the ghost is possessing the rogue's body (a living vessel for the time being), does the channeled energy damage the possessing ghost since it is an undead creature within range of the channel? Or does the channeled energy get harmlessly absorbed by the living shell of the rogue's body?