In the synopsis for the 100th episode of Supernatural, which will be airing on April 15, it mentions a "familiar face returning". If you're into spoilers, آپ probably know who this is.
At the end of Supernatural season 4, Sam turned his back on his brother, left him behind and went off to do what he thought needed to be done in order to stop Lilith. In "99 Problems," it was Dean's turn to do the same thing.