In SharePoint 2010, when trying to edit a page, you might get something close tothe following error:

Sys.InvalidOperationException: Could not find UpdatePanel with ID 
'{yourID}'. If it is being updated dynamically then it must be 
inside another UpdatePanel.

Most likely, someone was trying to update the page, and cancelled it before it was finished. Either way, the Edit Page function does not recognize the Unique ID for the UpdatePanel.

The easiest way to fix this is to go into SharePoint Designer, open the site, and check/view the history for that page. Simply restore to a previous version and that should fix it.