If the Telerik.Windows.Controls assembly is referenced from a local folder whilst the Telerik.Controls.Navigation.dll is referenced from the installation folder, as soon as a control from Navigation is added in a UserControl, a design-time exception is raised. The excpetion is "Error HRESULT E_FAIL has been returned from a call to a COM component." StackTrace: at MS.Internal.XcpImports.CheckHResult(UInt32 hr) at MS.Internal.XcpImports.Collection_InsertValue[T](PresentationFrameworkCollection`1 collection, UInt32 index, CValue value) at MS.Internal.XcpImports.Collection_InsertDependencyObject[T](PresentationFrameworkCollection`1 collection, UInt32 index, DependencyObject value) at System.Windows.PresentationFrameworkCollection`1.InsertDependencyObject(Int32 index, DependencyObject value) at System.Windows.Controls.UIElementCollection.InsertInternal(Int32 index, UIElement value)