"Sys is undefined" error because WebResource.axd is returning the wrong content
<p>I'm getting the dreaded "Microsoft JScript runtime error: 'Sys' is undefined" error while trying to develop a site in ASP.NET on .NET framework 4.0.  This was working fine until I tried programmatically adding items to a menu, and this started happening.  While debugging, I pointed Fiddler at my app and found that the request for the WebResource.axd is returning successfully (code == 200), but the response text is  <em>the whole html page</em> and <strong>NOT</strong> the javascript that it is supposed to be returning, and this is why Sys is undefined.</p>

<p>What's going on here?  Any help much appreciated.  Thanks.</p>
