First time ever seeing that, and wondering what happened there.
I logged out of a character, then went back to the login screen (I think, or maybe it happened on its own) and I got the message:
Your account has logged in somewhere else. If this comes as a surprise, you should change your password and check your security settings.
(BLZ51900060)
Well, I didn’t have several clients running or such and only log in from one system. But could this have been a glitch with another cause as the message indicates? I didn’t notice any other anomaly, except that since a day or two switching realms sometimes takes half a minute or so.