This is a quick change that is easily missed when adding AJAX functionality to an old ColdFusion site with CFAJAX tags. In my case, I was adding this functionality to a site that had originally been developed about ten years ago. This site runs on a Windows server with IIS.
After getting the site upgraded and working properly in the dev and test environments, I discovered that several errors were happening in prod that never happened in test or dev. The most common of these errors was “Coldfusion is undefined”.
Of all things, the Virtual Directory to CFIDE had never been created. As is so common, major problems can be caused by tiny bugs in code, or in this case, in the Web server configuration itself.
Create a Virtual Directory in the root of the website and name it CFIDE. It should, in most cases, point to C:\inetpub\wwwroot\CFIDE. Once this is done, the CFAJAX errors should be no more!
UPDATE via Gavin Pickin (@gpickin):