After executing cairo-dock (http://www.cairo-dock.org) in Linux / KDE 4.3 .2 (Kubuntu 9.10), all Qt based applications become unable to play videos.
I have tested kaffeine, gwenview, smplayer and dragon player, all with the same result: the area of the window where the video should be displayed shows only a transparent gray background (if desktop effects are on in KDE) or a solid black baground (if desktop effects are off). I can hear the video, and the video players show a progress bar running as if the video was being played, but no image is shown. Non-qt video players (xine, mplayer, gnome player) work flawlessly after running cairo-dock.
The Qt based video players work ok after executing cairo-dock if I set the XLIB_SKIP_ARGB_VISUALS environment variable to 1 (export XLIB_SKIP_ARGB_VISUALS=1) before running them, or if I log out KDE and log in again (and don't execute cairo-dock).
I don't know what cairo-dock does to trigger this bug, but certainly there are (or there will be) other applications that trigger it too.