This happens with both xmoto and supertux 2 (and probably others);
It's really easy to reproduce: Start game (let's say xmoto) - make sure it's windowed, move it around or open another application over it's window - parts of the game will still be where the game window isn't or shouldn't be.
Let me show you with a screenshot:
http://wocares.com/host/images/888678_screenshot.png
It's not really that big of a deal, but kind of annoying.


