If the DNS server of your local ISP maps rt.com into an IP address that it controls but for which the server has a certificate for a different domain, i.e. upc.

If the DNS server of your local ISP maps rt.com into an IP address that it controls but for which the server has a certificate for a different domain, i.e. upc.biz, there is a conflict: your browser still has rt.com in the address bar (it is not a http redirect) but it sees a certificate for upc.biz, so it reports a certificate error.