That's probably the closed captioning information you're seeing. Like static, but more organized? It just means you're seeing too much of the top of the frame. Adjust your TV to cut it off above the viewing area.

As it is, I sometimes see it, as I have my TV cutting really close on it, and one of the local TV stations shifts their whole frame down quite a bit every once in a while. I'm sure it's a bug on their end,