Yeah. You could think of that background as the canvas that the game paints itself to. The level is painted onto that background but when nothing is painted over what's already there it just retains the old image. This is why you'll see thirty or more little blocks going off into nowhere. Those (your level) appear with every frame and when there's nothing to render over it, they stay.

The reason you don't see that stuff happening inside of any typical level is because the level is always there to render over it. Per se. Where there aren't level blocks the game will have a sky.

That's why you're having problems. I didn't really solve the problem, but just thought you'd like to know that junk. Do what the people above said.