I think kaffeine is the problem, it sometimes locks at closing it, it is appareantly closed, but it is still running in memory (it is listed in ps output), after that, if you try to run kaffeine again to play another movie, it conflicts with the "still running" kaffeine and does not start. If you repeat this several times, it finally turns KDE crazy, doing hard to start a KDE application.
I am using GNOME since a few days, but i can remember that i have had this kind of problems with kaffeine recently using KDE + kaffeine from lenny packages.
I have to say that i also usually use Kubuntu and here i have never found these issues with Kaffeine, i think it is a Debian matter.
Anyway, i will turn to KDE 3.5.9 to check if i can reproduce this and another bugs.