Path.Data styling works only on first instance of styled object

Path.Fill is a DependencyProperty, while Path.Data isn't. Instead do:

<DataTemplate>    <Grid ...>        ...        <ContentPresenter Content="{StaticResource MyPath}"/>    </Grid></DataTemplate>

ContentPresenter.Content is a DependencyProperty so this should work:

<Path x:Key="MyPath" Margin="4" Style="{StaticResource ErrorIconPath}"       Stretch="Uniform" Width="26" Height="26" VerticalAlignment="Center"      RenderTransformOrigin="0.5,0.5" Grid.Column="1" Grid.Row="1"      UseLayoutRounding="False" HorizontalAlignment="Center"      Data="F1M874.094,289.369L854.3,254.63C854.028,254.151 853.515,253.856 852.958,253.856 852.403,253.856 851.89,254.151 851.617,254.63L831.824,289.369C831.555,289.84 831.559,290.416 831.835,290.883 832.111,291.348 832.618,291.634 833.165,291.634L872.752,291.634C873.299,291.634 873.805,291.348 874.081,290.883 874.357,290.416 874.361,289.84 874.094,289.369 M855.653,287.189L850.264,287.189 850.264,282.745 855.653,282.745 855.653,287.189z M855.653,279.41L850.264,279.41 850.264,266.077 855.653,266.077 855.653,279.41z"/>

I am guessing that Geometry cannot be shared. Have you tried setting the x:Shared= "false" to:

<Style x:Key="ErrorIconPath" TargetType="Path">

I've experienced the same behavior in Silverlight and asked a similar question here on StackOverflow.com ( https://stackoverflow.com/q/13426198/1796930), but as I'm writing this, it's been 1 month and I've yet to get even a single answer.

However, as you mentioned in your first edit, I too was able to perform a workaround by creating a resource with my geometry data as a string and then binding the Data property of the Path objects to the string resource resource.

I also had to create two instances of the Path objects that were identical other than each one using a different resource (i.e. two different icons) and then binding the visibility of each to a property in my ViewModel to display the appropriate one.