Here is a XAMLToy ready code sample:

<Grid
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<Rectangle Width="50" Height="50" Fill="DarkSeaGreen">
<Rectangle.Triggers>
<EventTrigger RoutedEvent="Grid.Loaded">
<BeginStoryboard>
<Storyboard FillBehavior="Stop" Storyboard.TargetProperty="Width">
<DoubleAnimation To="200" Duration="0:0:2"/>
</Storyboard>
</BeginStoryboard>
</EventTrigger>
</Rectangle.Triggers>
</Rectangle>
</Grid>

In the above sample the animation should be ended after its execution due to FillBehavior=Stop, but it instead will hold it as if its FillBehavior was HoldEnd.