While working on a new client application and ran into a bizarre issue with the WebResource.axd handler. The issue manifests itself as a JavaScript error when trying to do a PostBack from your WebForm.
In IE7 you'll get the standard Error pop-up message informing you that
'WebForm_PostBackOptions' is undefined
Not exactly the most useful error message. After a little JavaScript debugging in Firebug I found that the error was thrown because the WebForm_PostBackOptions method was missing. Yikes!
How is it missing?
As of asp.net 2.0, the Framework uses a new WebResource.axd handler to send assembly resources (like images, CSS files, JavaScript files, etc...) down to the browser. I fired up Fiddler and took a look at the WebResource.axd?d=xxx files sent down to the browser... and they were empty!
Yeah, the server was sending empty resource files down to the browser for all of the requests to the WebResource.axd handler.
After much Googling, debugging, and head-banging-against-the-desk... I had no solution. Lucky for me I work with a pretty smart guy and he was able to shed some light on the problem.
Check your httpModules
It turns out that I had configured a custom httpModule that was interfering with the WebResource.axd handler. After removing that pesky httpModule from my web.Config file all was well!
So, if you're seeing the WebResource.axd handler return empty resource files take a look at the httpModules and make sure they are all playing nice.