VS Designer trows exceptions if a RibbonTab is placed in a UserControl:  

If the RibbonTab control size isn't defined, the designer throws a System.InvalidOperationException - FrameworkElement_LayoutMeasure_InfinityNaNReturned.  

If the RibbonTab size is set, then clicking on the tab in the designer raises a System.ArgumentException - Property 'Items' was not found in the property collection 'Grid'.