I had the code base for my first WPF application ready to go, so I compiled a release version of software and tried to run it from the .exe instead of directly from Visual Studio.  I was promptly greeted with the following error:

The tag 'WindowsFormsHost' does not exist in XML namespace 'http://schemas.microsoft.com/winfx/2006/xaml/presentation'.

It ran flawlessly from the Visual Studio .NET debugger, which I found a bit odd.  I tracked the issue back to the following problem.  When I originally referenced the WindowsFormsIntegration.DLL from my project, the "Copy Local" option was set to false.  When I went back in and set that value to True, the application promptly broke when it ran in the debugger.  I don't mind when things break as long as they break consistently, so this was a step in the right direction.  I had found a few helpful forum entries informing me that you have to specify the xml namespace for the WindowsFormsIntegration assembly.  So I added the following xmlns entry:

xmlns:wfi="clr-namespace:System.Windows.Forms.Integration;assembly=WindowsFormsIntegration"

And changed my WindowsFormsIntegration tag from

<WindowsFormsHost>
    ...
</WindowsFormsHost>

to

<wfi:WindowsFormsHost>
    ...
</wfi:WindowsFormsHost>

and it started working just fine.