A quick peak at the schema tells us there are two columns in the preferences table that are unused:
edit_emails_off, which was never implemented
plain_text_skin, which may have been implemented at one point, but which has long since been replaced by skin_id and the ability to choose from many skins
These can be removed. Any reference to these columns in the preferences.yml fixture should also be removed.
A database admin should migrate up, down, and back up.