yeah but theres the vampire dean met in limbo, benny hitched a ride in deans body multiple times right? not exactly possession but yeah, close.

oh and yeah it was a dream.
"Dean encounters a future, demonic version of himself in a nightmare, which shocks him into starting to fight for his life. After waking up, Dean admits to Sam that he doesn't want to die. Meanwhile, Bela steals the Colt, meaning they won't have it when the hellhounds come for Dean."
- http://supernatural.wikia.com/wiki/Dean_Winchester

found it