First, this is because you copy/pasted bad code, either from Word or from elsewhere on the web OR because you edited some HTML and you forgot the closing tag, as you just did here at that last post. Don't do that.
To prevent harm from coming to your blog in future, go to Settings->General and select "wordpress should correct invalidly nested XHTML" and then to fix what's going on now, go to your most recent post and edit it somehow, any way. That will cause the correcting to kick in. Update the post. See if that solves the problem. If not, go to the previous post and edit that one, and so on till suddenly the problem is solved.