Failed to load resource: the server responded with a status of 503 (Service Unavailable) in heoku code example

Example: failed to load resource: the server responded with a status of 503 (service unavailable)

This specific problem is why packages like express-favicion and serve-favicon exist. They add a handler for the favicon.ico file.

What you've done there has only made things worse as it tells the client that the icon for the page is actually the page itself.