Troubleshooting XamlParseExceptions in Expression Blend 4

What is the best way to troubleshoot XamlParseExceptions in Blend?

I'm using Blend 4 RTM, with a SL4 project, and I get totally stumped wh开发者_如何学JAVAen I get the "Red Screen of Death":

"An Exception was thrown. XamlParseException: [Line: 0 Position: 0]..."

Is there a list of techniques that help? The error message is completely opaque and useless. Usually these kinds of errors creep up on me when I use the Toolkit, and try to style things like HeaderContentControl (derived classes actually), NumericUpDown, etc. Most recently I've been stuck with this last guy and have strip everything out... When I try viewing a UserControl that has an instance of this, I get the RSOD right away.

I've tried:

• Making sure that projects reference System.Windows.Controls, even if they don't apparently need it.

• Making sure I use relative paths to a central place where the April Toolkit is installed and use ....

• Make sure my styles are correctly set up with xmlns namespaces, etc.