See http://musicbrainz.org/release/a198a492-71b1-4d91-bb2c-c680a97936e5/edit when clicking on "Tracklist tab":

Caught exception in MusicBrainz::Server::Controller::ReleaseEditor::Edit->edit "Can't call method "add_error" on an undefined value at /home/musicbrainz/musicbrainz-server/script/../lib/MusicBrainz/Server/Form/ReleaseEditor/Tracklist.pm line 234."

Stack trace:

line 234 MusicBrainz::Server::Form::ReleaseEditor::Tracklist
line 105 MusicBrainz::Server::Form
line 524 MusicBrainz::Server::Wizard
line 238 MusicBrainz::Server::Wizard
line 173 MusicBrainz::Server::Wizard
line 388 MusicBrainz::Server::Wizard
line 132 MusicBrainz::Server::Wizard
line 91 MusicBrainz::Server::Wizard::ReleaseEditor
line 17 MusicBrainz::Server::Controller::ReleaseEditor::Edit
line 270 MusicBrainz::Server