Regulary, when opening any program in full screen (at least I think it only happens then) my whole plasma desktop seems to vanish and only a black screen is left. The panel is completely gone, as well as all the plasmoids and also the background picture.
Sometimes the plasma engine restarts itself, and everything appears again. Sometimes it doesnt and I have to restart the X server. That is really annoying.

I am using KDE 4.2.2.

Sometimes that happens to me too (but not very often). If plasma does not restart, you can press Alt+F2 and run "plasma" to start it back up.

Deleting the plasmarc and plasma-appsrc file fixed solved the problem.