I do know how to get around the whole html error message thing, though, because the same thing started happening to my Aigaion Girl blog one day, and I have to do this every time I post on there. Hopefully I'll be able to figure out how to get rid of it within the next couple of days. In the meantime, this is what you do:
When you try to post, an annoying error message appears. (click to enlarge)
Highlight everything up to the first p (it looks like this <, followed by a p)
Despite the fact that it says the title tag is the only thing that's wrong, taking care of that won't work, because it just puts the tag back when you try to post again.
Delete everything that's highlighted and publish the post. Everything should work swimmingly now.
Sorry about all this, guys. I have no idea why it suddenly started doing this, because I assure you that neither Rhiannon or I have been messing with any of the code lately, and of my 11 blogs, this one and the Aigaion Girl one are the only ones to have this error. Hopefully I'll get it sorted out soon.