Wikimedia Bugzilla is closed!

The configuration script emits an HTML form containing two input elements having the same name, DBmwschema.  Only one of these is visible to the user at a time, but both are active as far as the browser is concerned.  The result is that, when the form is posted, both the visible element (which the user will have changed to something more meaningful) and the invisible one (used by a database that the user doesn't have) are included in the POST body.  The install phase will see (in my experience) the second one, which still has the default value, and then fail.  Renaming the second element fixes the problem and allows the installation to proceed with the correct schema name.