CodingGorilla

        <Style TargetType="{x:Type TextBox}">
            <Setter Property="Validation.ErrorTemplate">
                <Setter.Value>
                    <ControlTemplate>
                        <Grid>
                            <Border BorderBrush="#FFCB2E2E" BorderThickness="1" Background="#11FF0000" IsHitTestVisible="False" x:Name="errorBorder"/>
                            <AdornedElementPlaceholder x:Name="placeholder" />
                            <Popup AllowsTransparency="True" HorizontalAlignment="Right" HorizontalOffset="0" VerticalOffset="0" PopupAnimation="Fade" Placement="Right" 
                                   PlacementTarget="{Binding ElementName=errorBorder}" IsOpen="{Binding ElementName=placeholder, Path=AdornedElement.IsFocused, Mode=OneWay}">
                                <StackPanel Orientation="Horizontal">