The Color Mix node produces Artifacts. In Black areas of the picture pixels from other frames are inserted. This happens in Blender 2.64 RC2, but not in the 2.63a release build and is also visible in the rendered movie.
I have attached an example .blend file where you may simply insert any movie file with black areas into the input node and scrub the timeline a bit to see the effect and a screenshot (There are some other nodes in there as the problem occured while grading).