After playing with references in your project, you may end up seeing your beloved Telerik controls in this annoying error box:
Error Creating Control – objectName
Failed to create designer ‘Telerik.Web.UI.RadDopDownList…’
This root cause of this issue is that a mismatch between the Telerik DLLs you referenced and the Telerik controls you have in your page.
In order to fix this issue, simply remove your Telerik references and add the ones whose version number match with the controls in your page.