title: System.UI.Web.ScriptManager is not allowed on this page. The type is not registered as safe error in SharePoint
body: <p>I've been getting an error in SharePoint that reads:</p>
  
  <p>System.Web.UI.ScriptManager is not allwoed on this page.  The type is not registered as safe.</p>
  
  <p>I googled around, and the results pointed to missing AJAX server extension installations while using the asp:scriptManager tag.  However upon closer inspection of my project, there was no findings of such tag.  Whats even more strange, was that after I reinstall/reset the masterpage, the error went away and I was able to read my site again.</p>
  
  <p>I was wondering whether the above error would appear if certain SharePoint files gets overriden?  Or are there other less obvious/non-trivial reasons for it?</p>
  
  <p>Thanks.</p>