Domains can end with a simple period. This simply makes it a FQDN, relative to the root zone (an absolute name). This is all according to spec, but, browsers (and other software as well) have for a long time simply assumed the domain to be equivalent (foo.com==foo.com.).
This is not usually a direct issue when we are browsing the web, however is something that must be kept in mind if writing systems that check validity of or use the Host-header to do internal product lookups.
References:
