Comment on Technical Support by SEO Dave.
Sounds like a code error, use the W3C Validator http://validator.w3.org/ to track down the code errors. Depending on your site you might have a lot of errors that don’t really cause serious problems, start looking for errors about closing /div> in particular.
Also see the comment above, if you are using an autoblog plugin you’ve probably scraped an article or something with broken code within it. If you have a post with just one extra closing /div> within it, it will break the layout of lots of WordPress themes.
Easiest way to track a bad post down is load them one at a time and see if they have the messed up layout, if they don’t that post is OK, if they do you’ve found the culprit. If all your posts are ‘broken’ it’s probably not a post, look at your plugins.