I was posting to my 100 Days of Blog microblog, which only has two categories: timeline, which is use to control what goes to the timeline (same as my main microblog), and a 100DaysofBlog category to control some cross-posting. The 100DaysofBlog category only exists on the 100 Days of Blog microblog, not my main microblog.
While writing the post, which I know was being published under 100 Days of Blog because I glanced at the line above the editor that tells you what blog you are publishing on, I switched to another tab to look something up under Account → Design → Edit Themes. Then went back to finishing the post and published it.
I was surprised to then discover that the post had published to my main microblog instead of the 100 Days of Blog one. Additionally, the 100DaysofBlog category had all of a sudden been added to my main microblog. I have since cleaned both of these up, by deleted the blog post, copying the content first, and recreating it on the correct blog, and deleting the category that was erroneously created on my main microblog. But this seems like a bug to me.
The only thing I can think is that by navigating to the other blog under Design somehow switched the in-progress post to publish to the other blog and that’s a bit disconcerting. If that is the case, I sure wish there was a way to move a blog post from one blog to another if you have multiple blogs and a premium account. I’m also a bit confused at the method that created the category on the main microblog without me actually creating it via the Category page.