This design time error is caused by having two instances of the Telerik.Web.UI assembly with different versions (might be Trial - Dev as well), loaded in the same Visual Studio AppDomain.
This could happen as a result of a reference update, e.g: a page with RadControls was loaded in design mode, the reference was updated and the page got reloaded again. This is a one-time only problem and its fix involves a Visual Studio restart.
Another possible reason for that is that the solution contains multiple projects, each referencing a different RadControls for ASP.NET AJAX assembly. Thus, loading a page with a RadControl from ProjectA in design mode and loading a page with a RadControl from ProjectB could result in the TypeCastException.
In that case please, update both the projects to use the same RadControls assembly, or split the projects to different solutions. Please, restart Visual Studio after any of the approaches.
the Telerik team