<ResourceDictionary>
<ControlTemplate x:Key="ErrorTemplate" TargetType="{x:Type Control}">
 <Grid>
  <Border
            Background="#11FF0000"
            BorderBrush="#FFFF0000"
            BorderThickness="1"
            IsHitTestVisible="False"/>
   <Polygon
            Fill="#FFFF0000"
            Points="0,0 10,0 10,10"
            HorizontalAlignment="Right"
            ToolTip="{Binding ElementName=adorner,
                              Path=AdornedElement.(Validation.Errors)[0].ErrorContent}"/>
                        <AdornedElementPlaceholder x:Name="adorner"/>
</Grid>
</ControlTemplate>
<Style TargetType="{x:Type TextBox}">
            <Setter Property="Validation.ErrorTemplate" Value="{StaticResource ErrorTemplate}"/>
</Style>
</ResourceDictionary>