In Windows NT 4.0, you used Control Panel / Network / Identification to leave one domain and join another. When you logon to the new domain, you are greeted with:

The system cannot log you on to this domain because the system's computer account in its primary domain is missing or the password on that account is incorrect.

The probable reason is that you waited more than 10 minutes to reboot.

During that time, the NetLogon service discovered that your secure channel password was the default password. NetLogon then changed the secure channel password, and recorded it on the old domain controller, since that is where your secure channel was established. When you finally rebooted, the new domain controller did not recognize your secure channel password, and it wasn't the default.

NOTE: This doesn't happen when you switch from a workgroup to a domain, because NetLogon isn't running.