ASP.Net AJAX Toolkit and 'AjaxControlToolkit' is undefined

If your site uses AJAX, and you receive the dreaded 'AjaxControlToolkit' is undefined error, this might help you.

After some troubleshooting with Fiddler, I found out that I installed the FlatCompress ISAPI filter a while ago to test HTTP compression. It turns out this causes some problems with the ASP.Net AJAX toolkit. After I disabled it and restarted IIS, the error went away. Great. Now I can continue testing BugNET...