The ErrorDocument rule in an Apache config lets you set the response content for various error codes, such as 404, 500, and so on.
If the ErrorDocument rule references an external URL (starting with "http(s)"), even if that is the same server the Apache instance is serving up, then the response for an error code is actually a 302.
docker build -t <name> .
docker run -d --rm --name=apache-test -p 80:80 --mount type=bind,source="$(pwd)"/src/,target=/usr/local/apache2/htdocs/ <name>
http://localhost will return the default Ember application static HTML page with some extra requests that should 404.
The HTML referenced in the iframe will fail with a 302 that then redirects to a 200 that serves up the 404 page. Confusing, right?!
The JS file will respond with a 404 as expected.