What you're seeing is an artifact of the defocus method being used. Doing defocus as a post process can only be an approximation as there is not enough information in an image + z-buffer to do it completely accurate, sometimes it has to guess. What you see in this case is that in one frame the background color is blurring over the cube, while in the next it doesn't. The Max Blur and Threshold parameters can be used to tweak the results.