I think I figured it out. I think that the issue crops up if your server sends a response header of ‘Content-Type’ set to ‘text/xml’ and you send no content. The XMLHTTPRequest object then says ‘oh it’s xml? fine – i’ll just PARSE THIS and store it in the reponseXML property.’ At that point, the XML Parser probably bombs and nothing catches it. But anyway that’s just a hunch and I’m only blogging about it because I’m bored. I’ll have test results available later.