Just had a new user create a subdomain called test.domain.tld, and upon attempting to create the database, Virtualmin errored out with:
Message was: "Failed to create virtual server : A MySQL database called test
or user called domain already exists"
Obviously a database test already exists, as it is the default database that MySQL creates on initialization. PostgreSQL also has template0 and template1.
These names should probably be special cased so that a user with one of these names gets a domain appendage automatically, or something.