Why do I have mce_serialized="1" showing on my website?

Many content management systems use the TinyMCE editor to allow for easier editing of content.  The editor allows you to create and edit html content without knowing html.   This works well, but occasionally, things can get messed up. 

I had an instance where every time the client saved a change, the page would show mce_serialized="1" in the browser.

I tracked this down to a missing <table> tag in the html code (I had moved it to a non-editable portion of the code by accident), and it was throwing off the editor apparently.  So if you are getting this message, check the html syntax of the data being edited - it's likely that something there is causing the problem.