I got this Error when I added ScriptManager on a "Site.Master" (Master Page)
All child webForm of site.Master that contains ScriptManager. produce this error.....
Only one instance of a ScriptManager can be added to the page.
help Thanks
use a ScriptManagerProxy on your child pages/user controls
http://msdn.microsoft.com/en-us/library/system.web.ui.scriptmanagerproxy(v=vs.110).aspx