Domain mapping question (hypothetical)

I just recently mapped my own domain to wordpress.com blog and there is no problem there.

However, I want to know the answer to these hypothetical questions that I have (I could not find answers on the web):

WordPress.com only allows domain mapping AFTER it checks that the domain is pointing to their domainservers. But what would happen if someone mapped my domain to their account AFTER I changed dominserver-info at my registrar and BEFORE I mapped them to my blog?

I know there is very very little possibility that it could happen, that is why the question is hypothetical.
And I know that at least one free-hosting company allows creating account for a domain name, even BEFORE the domain is pointing to their nameservers.

Of course, you can always go to another host (and change dominserver-info at you registrar).
But what would you do if you really wanted to be hosted at specific host, and someone already mapped your domain?