I've only run into a few times that I have had to use the " -t testing or -t unstable " install option, I've been playing around with and using debian for about a year or so and just discovered that here on this forum from using knoppix. I think you are correct in saying that with all three releases referenced knoppix is a "mixed bag of goodies" it must need to be that way or it wouldn't.

For example I upgraded fluxbox and to get to 1.13 (??) I had to specify -t unstable, later I found the maintainers site and used his URI in sources.list and went a step beyond sid.

The only places I've had problems is where a package exists in stable and requires KDE2 libs, etc. I use quanta for web pages and the only "official" debian quanta package is in stable, built for KDE 2 so it won't install, I have found debs of quanta 3 but I couldn't figure out the URI for sources.list so I used dpkg to install them, (WARNING in case you don't know, this can raise havock with your system, it is easy to upgrade a package to a non-functional version, std. linux liability release" If you break it you get to keep the pieces" been there, done that)

BTW, If you are looking for non-standard apt-get URI's take a look at apt-get.org there is a whole bunch of stuff there.

HTH

rock