When the ResourceManager kills an application, it leaves the proxy URL redirecting to the original tracking URL for the application even though the ApplicationMaster is no longer there to service it. It should redirect it somewhere more useful, like the RM's web page for the application, where the user can find that the application was killed and links to the AM logs.
In addition, sometimes the AM during teardown from the kill can attempt to unregister and provide an updated tracking URL, but unfortunately the RM has "forgotten" the AM due to the kill and refuses to process the unregistration. Instead it logs:
It should go ahead and process the unregistration to update the tracking URL since the application offered it.