first disc (cd) mistaken for last disc (= dvd)

[JonnyJD]

example:
http://musicbrainz.org/edit/19736151

When our discId is the last discId found for a release in the database, we guess this is the last disc for the release.
However, there can be another disc after that if that disc has no discId. This is always the case for (bonus) DVDs.

The frequent case CD+DVD is only affected if there are multiple discIds given for the first disc AND our discId is the last discId in the list.

[JonnyJD]

We have two possibilties:

1. We don't guess discs as beeing the last disc and ask for an offset (which makes submitting of 2-disc releases quite painful)
2. We still tell our guess (and what that means for the offset), but we do ask for the offset

[JonnyJD]

This is somehwat related to the old #16