nVidia, ATi differences.

I’ve got a couple of small opengl programs (learning exercises - recursive geometry generation, stencil buffer, primitive stencil shadows).

They were developed, and run fine on my nVidia (GeForce4 Ti4200, Fedora Core2 x86).

However, I encounter rendering issues on my laptop (Mobility Radeon 9700, Ubuntu Breezy AMD64, fglrx driver).

Some things I can recognise (with ATi, when a window is moved the buffers maintain their absolute position within the screen, with nVidia, they remain relative to the window).

Others currently have me baffled, as I am unable to spot the cause. What other differences are there between how the two implementations behave?