If Possession is played while executing another Possession, who takes the next turn?
If Bob plays Possession, even if he is currently being possessed by Alex, Bob will control Carl's next turn.

This basically means if you are possessing someone, you should not have them play Possession.  Especially in a 2 player game.

(Note that sometimess you can't avoid playing a Possession - for example if you play a Golem, you might end up being required to play Possession as well.)