Let me explain what happened. I added a new release: http://musicbrainz.org/release/caa7cb2d-d782-4b66-8c45-738724f3d883
Since it's a new version of http://musicbrainz.org/release/caa7cb2d-d782-4b66-8c45-738724f3d883 with additional tracks (the original release only had 9 tracks) and uses the original recordings (remastered) for the first nine tracks, I assigned those first nine tracks to the same recordings and assigned the other tracks to their sources.
Then I created an edit http://musicbrainz.org/edit/14468425 to merge the release groups of the existing release and the new one.
I set a remaster AR between the two: http://musicbrainz.org/edit/14468428
Then I added the DiscID http://musicbrainz.org/edit/14468434 (which no longer seems to be auto-added when I create a new release via Picard's lookup).
Then I started doing a little artist:recording AR and then I saw that there were no Works assigned to these recordings, so I started making those.
I noticed some errors in the spelling/spacing of the original release's tracklist, so I fix them: http://musicbrainz.org/edit/14468636
Sometime (briefly) after doing that, I noticed that the original release no longer had its own tracklist but copied the tracklist of the new release I added. So I tried to manually fix it by removing the extra tracks: http://musicbrainz.org/edit/14468660
This had no effect, despite MusicBrainz seeming to let the edit through. The strange part is that it only affected the one original release and not the translated tracklist http://musicbrainz.org/release/65e32a17-8be3-4ed6-b9c0-7893d381b5ae (which also has a CD medium--it probably shouldn't, since it's a pseudo-release, I'll fix that later). It also didn't affect the otherwise shared recordings with its cassette cousin http://musicbrainz.org/release/279051ec-ae5b-3da5-8e0d-82550b045a35 although I noticed that I didn't fix the track titles on the cassette, so the problem might actually occur if the track titles match on the tracklists.
Either way, the problem seems to override the DiscID of the 9 track original release, so I imagine it's serious, especially if other editors trigger it to occur on other releases as they go about correcting them and adding new versions of those releases.