Oh! The main point of the whole thing!!!! Dean’s fear about Sam possibly going “dark” with his powers. In his fear-induced state when he sees Sam with yellow eyes…and when they flash again at the end, AFTER he’s out from under the ghost sickness???? Bobby did say that illness tends to hang around, so I guess that’s what that was, but I remember on first watch I thought “Oh crap…now Dean has some kind of insight or something.” But it was really just Dean’s fears manifesting. Very effective and unsettling though. Made me start to wonder a bit about Sam on first watch. And then Dean, of course, says NOTHING to Sam about seeing that; saying everything is fine. Ugh.