What happens if the subdomain the user enters is already taken?

What happens if, during the sign up process when the user enters the subdomain they want for their site, they enter something that's already taken?

I was hoping the add-on would check with the WP database to see if the site name is already taken, but it doesn't seem to. So what happens after the person orders if they've selected a username that's already taken? Will it mess up the original account? Or will the new account just not work?