Just spent a happy hour or so debugging misleading error messages to find out that Microsoft requires an obscure second action to activate custom error pages in addition to setting a custom error page. To serve a custom page on an error (in my case, 404), you go to the IIS control panel, select the site and double click on the icon marked Error Pages. This will give you a nice list of the usual error messages. You can double click on one of them and, using the handy browse button, select your new error page. Then you can go to a web browser and induce an error (in my case, typing the domain with a gibberish page name). You may be rewarded with a 500 error message that says that you cannot have an absolute file path in website.config, or some such (sorry I didn't get it exactly). You will look at the path name, read other path names, and eventually change the "C:" to "%SystemDrive%". This may reward you with more complicated error messages. Eventually, it will give you a 404. Hooray, that's what I was looking for. Except that it's the standard page. You may spend more time trying to figure out how you misrepresented the file name. You may notice the Feature Settings button and try it. Putting the file path into its Custom Page field won't work either. It turns out that you need to click the radio in the Feature Settings that says, "Custom Page." Then it will pay attention to the fact that you entered a custom page already. Do I sound bitter?