When a user logs onto a machine that is a member of a domain, the machine stores a copy of the username/password - known as "cached credentials". When the user next logs on, if the domain controller for the domain cannot be contacted (because, say, the machine is a laptop and the user is on a bus), the machine uses the cached credentials to authenticate the user. By removing the machine from the domain, the cached credentials are lost and only local accounts will have access to the machine. In short, you stuffed it
Oh and the domain admin need never know - each domain user can add upto 10 devices to the domain. After that, you receive a message saying that you've exceeded the number. Should be able to re-add the machine without trouble. Does depend on the setup of the domain though.