from http://bugs.musicbrainz.org/ticket/2494 (see comments there)

For a disc not found via TOC/disc ID, Picard submits ALL tracks including data, while apparent MB policy is to ignore such tracks. In other words, you can't match a disc to an existing MB entry without a workaround.

Recent examples include Clipse - Lord Willin' (extensive discussion here http://musicbrainz.org/show/edit/?editid=6512990); Death in Vegas - Scorpio Rising; Dexys Midnight Runners - Searching for the Young Soul Rebels; numerous CD singles that include video or multimedia tracks.