It's probably worth pointing out that you can't rely on the behaviour of backbuffers & frontbuffers on flip for different setups. That is, some graphics cards will swap buffers on a flip, others will copy from the back to front buffer (as you had expected).
You should *always* start from scratch every frame (i.e. Cls the backbuffer), or use triple-buffering and draw to your very own buffer as sswift suggested. That way you know exactly what's on screen on everyone's system.
You should *always* start from scratch every frame (i.e. Cls the backbuffer), or use triple-buffering and draw to your very own buffer as sswift suggested. That way you know exactly what's on screen on everyone's system.