Graphics on 3D use a mesh but what about graphics behind 3D?

I never experienced any troubles using 3D together with 2D. I heared there is a small percentage of old card that don't like the both together. But it shurely doesn't matter if you draw 2D on 3D or 3D on 2D or even 3D on 2D on 3D on 2D (afaik).