Generic.xaml

 <Style TargetType="local:NumericTextBox">
   <Style.Resources>
     <ControlTemplate x:Key="ValidationErrorTemplate1">
       <StackPanel>
         <Border BorderBrush="Red"
                 BorderThickness="1"
                 HorizontalAlignment="Left">

           <!-- Placeholder for the NumericTextBox itself -->
           <AdornedElementPlaceholder x:Name="AdornedElement" />
         </Border>

         <Border Background="White"
                 BorderBrush="Red"
                 Padding="4"
                 BorderThickness="1"
                 HorizontalAlignment="Left">
           <ItemsControl ItemsSource="{Binding}"
                         HorizontalAlignment="Left">
             <ItemsControl.ItemTemplate>
               <DataTemplate>
                 <TextBlock Text="{Binding ErrorContent}"
                            Foreground="Red" />
               </DataTemplate>
             </ItemsControl.ItemTemplate>
           </ItemsControl>
         </Border>
       </StackPanel>
     </ControlTemplate>
   </Style.Resources>
   <Setter Property="Background"
           Value="{x:Static SystemColors.ControlLightBrush}" />
   <Setter Property="BorderBrush"
           Value="{x:Static SystemColors.ActiveBorderBrush}" />
   <Setter Property="BorderThickness"
           Value="1" />
   <Setter Property="Validation.ErrorTemplate"
           Value="{StaticResource ValidationErrorTemplate1}" />
   <Setter Property="Template">
     <Setter.Value>
       <ControlTemplate TargetType="local:NumericTextBox">
         <Border Background="{TemplateBinding Background}"
                 BorderBrush="{TemplateBinding BorderBrush}"
                 BorderThickness="{TemplateBinding BorderThickness}"
                 Padding="{TemplateBinding Padding}">
           <VisualStateManager.VisualStateGroups>
             <VisualStateGroup x:Name="CommonStates">
               <VisualState x:Name="Normal" />
               <VisualState x:Name="Disabled">
                 <Storyboard>
                   <ColorAnimationUsingKeyFrames Storyboard.TargetName="Border"
                                                 Storyboard.TargetProperty="(Panel.Background).
                   (SolidColorBrush.Color)">
                     <EasingColorKeyFrame KeyTime="0"
                                          Value="{x:Static SystemColors.InactiveBorderColor}" />
                   </ColorAnimationUsingKeyFrames>
                 </Storyboard>
               </VisualState>
               <VisualState x:Name="ReadOnly" />
               <VisualState x:Name="MouseOver" />
             </VisualStateGroup>
           </VisualStateManager.VisualStateGroups>

           <ScrollViewer Margin="0"
                         x:Name="PART_ContentHost" />
         </Border>
       </ControlTemplate>
     </Setter.Value>
   </Setter>
 </Style>