Trying to log in to a different server than your home region.
Your account may be registered in NA, but you're trying to log in to Asia or EU server: error 12.
Is this a correct statement?
If so, it would explain why there are so many of these. Of course, the Asian servers are run by a different company (or two) than Blizzard.
Global play, as Blizz has already stated, will be available some time after 12:01 am PDT.