Just ran across a comment from Daniel Puzey here, with the following excellent advise:

Often, by default, you'll get an error reported at Line 1 of the xaml, which is an obvious lie. You can catch the original exception, though:

- Open the "Exceptions" window (Debug/Exceptions) in Visual Studio.
- Click "add"
- Add "System.Windows.Markup.XamlParseException"
- Check the box to break on throw for this exception.
- Hit F5!

You'll find that the XamlParseException you catch is much more descriptive, and will give the correct position in the xaml file.