How do I know what went wrong with an XHR request?

I am using mootools to send an XHR request. To a url that in other places I call using XHR is working fine. The server side 开发者_JAVA技巧does what it is suppose to do.

But,

I do not see any response headers.

The onFailure event is being fired and the XHR object is being returned to the event handler.

How do I debug this and see any kind of error message?

Header info and errors message I look for with Firebug and the native error console...NOTHING.

Maybe the serverside script runs into an exception before answering to the request. Check the error log for your website for entries. If you use PHP turn on error reporting in php.ini, so you perhaps get an answer from the server with information on the failure.