As Homer Simpson would say, DOH!

I forgot to change /etc/apt/apt.conf.d/00knoppix from
Code:
APT::Default-Release "stable";
to "testing" .

So, during upgrades, dependency resolutions of not-installed stable packages got a preference of 990, while not-installed testing/experimental packages got a preference of 500 and installed testing/experimental packages, 100.

That sure will put a damper on upgrades.