I have recently encountered a problem with a red checker pattern flickering over video when I am using VLC. The way to fix this is to change the video mode of VLC. I have read that you can switch it to "X11" output mode by going to Tools->Preferences->Video, then in the "Output" drop down, select "X11 Video Output". You will need to restart any video you are playing for the change to take effect. I also have tried the "OpenGL Video Output" option successfully.

The cause of this is a conflict between the video rendering in VLC and the compositing effects in KDE 4.x. You could also disable advanced desktop effects to resolve the issue, but who would want to do that when there is another option?