Read original post by Peter Kuhn at Mister Goodcat
Every now and then I want my text boxes in a Windows Phone project to show validation errors by using the magic that is provided by the binding engine and supporting interfaces like INotifyDataErrorInfo. When you try to do that as you would in e.g. WPF or Silverlight, the visual result is simply – nothing. Luckily, this irritating behavior is just caused by the involved default control template of the control on Windows Phone. All the infrastructure and the required logic in the control code are there, it' just that nothing is displayed because the required visual states are missing (for performance reasons).