You may encounter an unpleasant surprise when editing FlowDocument XAML (see http://msdn2.microsoft.com/en-us/library/aa970786.aspx for background on FlowDocument content).

If in, the FlowDocument XAML that follows, you attempt to insert a TextBlock (or any element not inline-derived) between the Run-element lines (Line 1 Line 2) then Visual Studio will just vanish without warning.   Expression Blend does not quit, but it does not complain about a TextBlock as an invalid child of a Paragraph...

  
<Window x:Class="Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window1" Height="300" Width="300">
    <Grid>
        <RichTextBox Margin="10,10,12,13" Name="RichTextBox1" >
            <FlowDocument>
                <Paragraph>
                    <Run>Line 1</Run>
                    <Run>Line 2</Run>
                </Paragraph>
                <BlockUIContainer>
                    <TextBlock></TextBlock>
                </BlockUIContainer>
            </FlowDocument>
        </RichTextBox>
    </Grid>
</Window>