While migrating a machine that does not think its master you get:

VACUUM
WARNING: skipping "pg_authid" — only superuser can vacuum it
WARNING: skipping "pg_database" — only superuser can vacuum it
WARNING: skipping "pg_db_role_setting" — only superuser can vacuum it
WARNING: skipping "pg_tablespace" — only superuser can vacuum it
WARNING: skipping "pg_pltemplate" — only superuser can vacuum it
WARNING: skipping "pg_auth_members" — only superuser can vacuum it
WARNING: skipping "pg_shdepend" — only superuser can vacuum it
WARNING: skipping "pg_shdescription" — only superuser can vacuum it
VACUUM
Export of temporary migration tables
Fri May 13 21:24:14 2011 : Exporting to /tmp/mbexport-SX54ZV
Table Rows est% rows/sec
tmp_release_album 991545 100% 125118 7.92 sec
tmp_recording_merge 640576 100% 138556 4.62 sec
tmp_work_merge 10552 100% 123470 0.09 sec
tmp_url_merge 1048 100% 48044 0.02 sec
tmp_release_merge 60052 100% 143105 0.42 sec
Failed query:
'SELECT COUNT FROM dbmirror_pending'
()
DBD::Pg::st execute failed: ERROR: relation "dbmirror_pending" does not exist
LINE 1: SELECT COUNT FROM dbmirror_pending
^ at /home/robert/musicbrainz/musicbrainz-server/admin/../lib/Sql.pm line 394.

at /home/robert/musicbrainz/musicbrainz-server/admin/../lib/Sql.pm line 406
Sql::_ANON_('DBD::Pg::st execute failed: ERROR: relation "dbmirror_pendin...') called at /usr/local/lib/perl5/site_perl/5.8.9/Try/Tiny.pm line 100
Try::Tiny::try('CODE(0x292af70)', 'Try::Tiny::Catch=REF(0x22ea110)') called at /home/robert/musicbrainz/musicbrainz-server/admin/../lib/Sql.pm line 409
Sql::_select_list('Sql=HASH(0x22e9790)', 'SELECT COUNT FROM dbmirror_pending', 'ARRAY(0x22ea000)', 'arrayref', 'CODE(0x27afb20)') called at /home/robert/musicbrainz/musicbrainz-server/admin/../lib/Sql.pm line 428
Sql::select_list_of_lists('Sql=HASH(0x22e9790)', 'SELECT COUNT FROM dbmirror_pending') called at /home/robert/musicbrainz/musicbrainz-server/admin/../lib/Sql.pm line 362
Sql::select_single_column_array('Sql=HASH(0x22e9790)', 'SELECT COUNT FROM dbmirror_pending') called at /home/robert/musicbrainz/musicbrainz-server/admin/../lib/Sql.pm line 377
Sql::select_single_value('Sql=HASH(0x22e9790)', 'SELECT COUNT FROM dbmirror_pending') called at ./admin/ExportAllTables line 640
DBD::Pg::st execute failed: ERROR: relation "dbmirror_pending" does not exist
LINE 1: SELECT COUNT FROM dbmirror_pending
^ at /home/robert/musicbrainz/musicbrainz-server/admin/../lib/Sql.pm line 394.
at /home/robert/musicbrainz/musicbrainz-server/admin/../lib/Sql.pm line 408
Sql::_ANON_('DBD::Pg::st execute failed: ERROR: relation "dbmirror_pendin...') called at /usr/local/lib/perl5/site_perl/5.8.9/Try/Tiny.pm line 100
Try::Tiny::try('CODE(0x292af70)', 'Try::Tiny::Catch=REF(0x22ea110)') called at /home/robert/musicbrainz/musicbrainz-server/admin/../lib/Sql.pm line 409
Sql::_select_list('Sql=HASH(0x22e9790)', 'SELECT COUNT FROM dbmirror_pending', 'ARRAY(0x22ea000)', 'arrayref', 'CODE(0x27afb20)') called at /home/robert/musicbrainz/musicbrainz-server/admin/../lib/Sql.pm line 428
Sql::select_list_of_lists('Sql=HASH(0x22e9790)', 'SELECT COUNT FROM dbmirror_pending') called at /home/robert/musicbrainz/musicbrainz-server/admin/../lib/Sql.pm line 362
Sql::select_single_column_array('Sql=HASH(0x22e9790)', 'SELECT COUNT FROM dbmirror_pending') called at /home/robert/musicbrainz/musicbrainz-server/admin/../lib/Sql.pm line 377
Sql::select_single_value('Sql=HASH(0x22e9790)', 'SELECT COUNT FROM dbmirror_pending') called at ./admin/ExportAllTables line 640

because the dbmirror tables do not exist yet.