As of iTunes 9, if you sort by Album by Artist or Album by Year, discs are no longer listed sequentially, so for example, if you have a 3-disc album, track 1 from every disc will be listed together, then track 2 of every disc, etc. This bug isn’t there if you just sort by album. Very annoying for those of us who like live albums (as they tend to be the most common multi-disc albums).