In many cases, developers are trusting the HTTP Host header value and using it to generate links, import scripts and even generate password resets links with its value. This is a very bad idea, because the HTTP Host header can be controlled by an attacker. This can be exploited using web-cache poisoning and by abusing alternative channels like password reset emails.
The web application should use the SERVER_NAME instead of the Host header. It should also create a dummy vhost that catches all requests with unrecognized Host headers. This can also be done under Nginx by specifying a non-wildcard SERVER_NAME, and under Apache by using a non-wildcard serverName and turning the UseCanonicalName directive on. Consult references for detailed information.