Several months ago, I took a leap into the future, and upgraded this blog to use a "Responsive" class template. There are four "Responsive" templates - Contempo, Soho, Emporio, and Notable. The differences between the four include more than merely the visible design.
Both the historically infamous "Another blog ..", and the currently infamous "Error 12", are part of the custom domain publishing process. Similar to the mysterious "bX" codes being an enhanced version of "We're sorry, but we were unable to complete your request.", "Error 12" (and variants) is an enhanced version of the monolithic "Another blog or Google Site is already using this address.". "Another blog" (which was generally seen during use of the Blogger Publishing wizard) is complemented by the equally annoying "Not found" (which is generally seen after Publishing is used).