InfoPath says "Reference to undeclared namespace prefix" when changing view in the designer

For some reason InfoPath 2003 SP1 told me I had a "Reference to undeclared namespace prefix" in one of my views after I added a node from deep within my message schema.

The namespace in question was indeed declared in the schema (which is valid), but for some reason it was not included in the XSL generated by InfoPath. I extracted the form files (from the File Menu) appended the missing namespace in the xsl file for the view and it got going again.