Here's the culprit:
http://musicbrainz.org/release-group/b649891f-3eac-3d9c-a040-6b5fa470beff

This was imported with only one release event, the 1980 release, which had no format assigned. I created the CD issue and was going to move the DiscIDs to that so we could set the 1980 release to vinyl without just deleting the IDs outright.
1) Select "Move" for a DiscID.
2) Enter the release name, "Live", and click the Search button.
– (2a. Believe it or not, there is more than one release named "Live" in the database, so the list might be a little long. I've also run into situations where the search generates a different list of releases every time, so you might have to search multiple times before the release you want shows up. Also, since the list just gives title and artist, both issues are displayed without an obvious indication of which is which. The easiest way to deal with it is to mouse over the release titles and look for the desired ID, which in this case begins with "b2fe".
3) Select that radio button, then click the Select button.

RESULT:
Bad Request
Sorry, there was a problem with your request.
Error message: The medium you are trying to attach a disc ID to belongs to a different release

It's also worth noting that not only is this edit between 2x sets, but each medium in the set contains the same number of tracks. I'm not sure if the bug occurs with all multi-disc sets or specifically in cases where the destination release has more than one potential target medium for the DiscID.