Back in the days when manga collections were larger, more expensive ($15 - $17) and flipped, I purchased the complete run of one of my favorites, Rumiko Takahashi's Maison Ikkoku.
When Viz started to re-released some of their older titles in smaller, less expensive and unflipped editions, I assumed that the contents were the same between the two editions. However, according to this page on Gilles Poitras's site, this is not the case. In particular, there seem to be four stories in the first volume of the second edition that were not in the first edition at all. With that being the case and the fact that the new editions are more normalized in their page counts, the old and new editions don't seem to correspond at all. I wonder if this is true of all of Viz's rereleases? If so, this would seem to be a detriment to those who may only have a partial run of the old collections, but doesnt not want to go back and repurchase everything. (I also note that many of the old editions are still available via Amazon, which is sure to confuse people...)