There's two things I've seen. Realm down, and Unable to connect.

I've avoided both of these by basically making sure I'm in the game for at least 60 seconds. I've successfully muled/mf'd this way, switching to all 16 characters (2 accts x 8 chars). It's after you exit a game within 60 seconds a few times (not sure of the number) that you get either one of the two messages, from what I can tell.