sub domains generally replace the 'www' default, as the www is the primary domain pointer, most domain names can be reached without adding the www prefix for example http://x.com and this will point directly to the same location adding the www.
Would show a different site, or would error 404 resource not found for a site root that contains nothing / doesn't exist.
i think whats happening with yours is that the sub you have created is pointing to the same location on the server as your main domain, and its looping back on itself thus creating the problem you are getting.
HMMM k its li this
instead of mail.yahoo.com
then it says server not found but i want it to take me to a error page of yahoo.com n say the url typed is wrong.... i already have an eroor page which is linked if my url is wrong after .com but between http://x.y.com/ it does lead me to an error page
Unique Error 404 Pages are something you need to talk to your hosting provider about, IF they allow you to create your own then it should be a simple case of having the custom page in the root of your domain. Depending how their services are set up, you may or may not be able to do this.