When fetching track data from MusicBrainz for a CD in multi-CD release, Rhythmbox ignores the divisions between CDs. Essentially, a multi-CD release is treated by Rhythmbox as one huge CD containing all of the tracks in CD 1, followed by all of the tracks in CD 2, and so on, and Rhythmbox fetches the track data (both title and length) starting with track 1 of CD 1 (no matter which CD is inserted) and proceeding through the list until it runs out of tracks on the CD. So, for example, if CD 1 has 8 tracks and CD 2 has 10, then when you insert CD 2 Rhtyhmbox displays a tracklist consisting of tracks 1-8 of CD 1 followed by tracks 1 and 2 of CD 2, instead of a tracklist consisting of tracks 1-10 of CD 2 (the tracks actually on the CD).

I discovered this when trying to play disc 5 of "Zoltán Kocsis Plays Bartók" <http://musicbrainz.org/release/a5f49088-adee-4660-af94-fa7db50e4d1d> (books I-III of "Mikrokosmos"). The tracklist should have started with CD 5 track 1 "Mikrokosmos, BB 105 (Sz. 107), Book I: 1. Melodie im unisono (1)" and proceeded to CD 5 track 96 "Mikrokosmos, BB 105 (Sz. 107), Book III: 96. Stockungen" according to MusicBrainz data. Instead the first track was listed as CD 1 track 1 "14 Bagatelles, BB 50, Sz. 38 (Op. 6): I. Molto sostenuto" and the 96th as CD 4 track 20 "Petite Suite, BB 113 (Sz. 105): III. Whirling Dance". Not only was it tedious to rename all 96 tracks, but since the track lengths were also incorrect and many of the Mikrokosmos pieces are quite short, this resulted in most of the tracks not scrobbling on Last.FM since the actual tracks ended long before Rhythmbox's track progress widget reached its midpoint.

I confirmed this with a set of the complete Beethoven symphonies <http://musicbrainz.org/release/1e6ec4d8-3a84-39b3-a8c9-b1306f1c5f86>. I inserted CD 2 and got the same tracklist as CD 1 (both CDs have 8 tracks).