Here's the danger of buying TV show DVD sets season by season: for many series, the studio will probably release a collector's edition set, which will include not only all the DVDs, but also extras that weren't on the individual sets.

That's what's happening with M*A*S*H. The same day that FOX announces that they will release the last season of the show in November, they also announce that they are releasing The M*A*S*H Collector's Set, which wll include extras "not  seen anywhere else!" So, if you want those extras, you now have to buy both. Or, if you're tricky, sell your individual sets for some cash and put it towards the new set.

Of course, even that might not be smart, because there might be extras on those individual sets that won't be on the collector's set! Oh, these studios.