Request http://test.musicbrainz.org/ws/2/collection using the credentials of an account with no collections, and it will give a stack trace.
Caught exception in MusicBrainz::Server::Controller::WS::2::Collection->list_list "Failed query:
'SELECT id, coalesce(
WHERE collection = col.id), 0)
) col (id)'
DBD::Pg::st execute failed: ERROR: syntax error at or near ")"
LINE 7: ) col (id)
^ at /home/musicbrainz/musicbrainz-server/script/../lib/Sql.pm line 79.
I haven't tested if this also happens on the main server (I don't have any alt accounts).