If you have been copying and pasting text from elsewhere without using either icon 5 or icon 6 in Row 2 of the visual editor, please see here http://en.support.wordpress.com/visual-editor/#pasting-text In short, copying and pasting from miscrosoft WORD which is not web compliant into your blog without using the correct process results in a mess. Typically this is caused by an open HTML tag in one of your posts.
Go to Settings > Writing and select “ ___ WordPress should correct invalidly nested XHTML automatically” and then scroll down and click “Save Changes.”
Posts > All Posts, change all your latest posts to Draft, click Update, change them back to Published, click Update again. (You can edit all of them at once: tick the square in front of their titles then select Edit from the Bulk Actions dropdown and click Apply.)
See > Themes > Layout Issues > Sidebar