Everybody's been talking about that scene where Cas tries to heal the wound in Sam's shoulder & Sam gets a brief vision, colored in red. Cas can't heal the wound and Sam is physically stunned by what he saw, but doesn't mention it to Cas or later to Dean.
Here's what happens:
Dean: "Sammy, please."
Dean w/a beard & wearing a scarf falls to the floor in the Bunker (the same version of Dean we saw fighting in the Bunker hallway in the Last Ride video).
Evil-looking Sam tilts his head to the side & it looks like his eyes go black
Gah! I can't wait to see where this is leading!!!