Causes need for MBS-1929.

Steps to reproduce:
Click "Add release" on an artist.
Add a title.
Go to Tracklist tab. Note that clicking the red X here does nothing - you can't remove the single disc.
Click "Add disc"
On dialog that pops up, click "Add disc" again on manual entry.
Now you have two discs.
Click the red X on both of them. It lets you remove both now.
Go to Edit Note tab. No validation failure
Submit edit
>>>> zero-medium release in DB.

http://musicbrainz.org/edit/14478672