Probably a minor issue: I added the main disc to a leftover "(bonus disc)" release, i.e. adding the new medium as medium 1 and moving the existing medium from position 1 to position 2. The new medium, however, is displayed as being moved from position 3 to 1.

See the attached screenshot for the edit review.

Editing history: http://musicbrainz.org/release/6af59b09-2f3c-46d3-b076-45ecfec17904/edits