Exceptions are occuring on ALL bindings, not just with RelativeSource

Command="{Binding AddCommand}"

will also raise an exception.

If you define two UserControls in a project and you add two RadTabControls - one RadTabControl per UserControl, then in design-time you can only change the selection of one of the tab components.

When the SelectedIndex property is read during animation the CLR wrapper will throw InvalidCastException because of invalid cast to int. The object returned from GetValue is DeferredReference object.