Custom error pages in IIS

When creating a custom error page, you need to remember to update the feature settings within IIS.
First, set your error page for the correct status code. e.g. 404
Set IIS either to execute a URL or respond with a 302 redirect.
Then on the right hand action bar, click Edit Feature settings and select the option to use the custom error pages. If this is not done, IIS will still use the default error page.