After reaching 49th level, I finally decided to start the Civil War quest, siding with the Stormcloaks. I completed the quest and swept those Imperial thugs off the map.

I decided to visit Blue Palace a few days later to see how good ol' Elisif and her crazy, widow eyebrows were doing. After a brief chat with Falk Firebeard, I heard the clang of swords in the distance and then all of the NPCs ran off to join a fight.

I followed along thinking that some leftover Imperials had busted into the castle to seek a last bit of revenge, but no. It was Faleen, Jarl Igmund, and Raerek. They were all there in Blue Castle--and hostile to me and everyone else. I stood by and watched the Stormcloak soldiers, Firebeard, Elisif, and even a nearby cook hack them up. But the hostiles have the "essential" flag set, so they can't be killed.

What the heck is going on? I assume this is a bug. A very entertaining bug, but a bug nonetheless.