I encountered this error after editing a web part (.aspx) page in SharePoint Designer 2013. When I looked at the code in SPD, an H1 element near the top of the page was highlighted yellow. Hovering over that indicated that SharePoint:AjaxDelta was not closed before the H1. Adding the </SharePoint:AjaxDelta>
fixed it.
Weird because it appeared SPD introduced the error after I was working on listview web parts or a page viewer web part elsewhere on the page.