Should be a bug report, but the movie demonstrates it nicer than plaintext.

To clarify: This movie has not been artificially constructed beyond the point of mixing 'the way it should look' with 'the way it looks on my character'. No game modding nor (3D) editing took place.
This account's character is somehow still in the Canthan New Year 'server' even though that event has long since ended. Come Halloween, the props changed to the Halloween ones (including The Mad King, with all its scripting), while leaving the decorations in place.
I don't really play with this char much, which is why it's still in this state. And yes, I exited Guild Wars numerous times since then, it's not like I'm running an ancient build of the game, nor did I leave my PC on for lengths at a time.
