Entering an unresolved artist in a recording's artist credits causes an internal server error.
This came up in http://tickets.musicbrainz.org/browse/MBS-3207 but I've also had it when hit with a case where the needed artist wasn't in the database.
If you enter a non-existent artist into the artist credits during editing a recording (causing the field to go red), submitting the change will cause an internal server error. In contrast, on a tracklist edit, this brings up the add missing entities page. We need something similar for recordings.