This issue implies Go v1.4.2 on darwin/amd64 architecture.

Currently, http.NotFoundHandler, that replies to all 404 requests has the following implementation:

// NotFound replies to the request with an HTTP 404 not found error.
func NotFound(w ResponseWriter, r *Request) { Error(w, "404 page not found", StatusNotFound) }

// NotFoundHandler returns a simple request handler
// that replies to each request with a ``404 page not found'' reply.
func NotFoundHandler() Handler { return HandlerFunc(NotFound) }
//   ^^^^^^^^^^^^^^^

This works pretty great for a helloworld application, but in the wild you would need to override default 404 page with a custom one. In the meantime, implementing your own 404 handler is not an issue at all, unless it's being called by FileServer() or ServeFile, which people are using to serve some static.

In my opinion, http.NotFoundHandler is not flexible enough and we must have a way to override it with custom 404 error handler.