You would be surprised how many movies on dvd/bluray have junk in the margins. Usually just a line or an overscan of the audio track. But a lot of them have it.
I turn off overscan as those artifacts do not bother me.
Sometimes you can see the closed captions line of the NTSC signal at the top of the frame of the video when watching an old show converted to digital from an NTSC source. It looks like a single line of black and white dashes which dances around quickly every time the onscreen captions would have changed and then sits static until the captions update again.