When using the OpenGL from the Radeon 7500 DRI free driver, i get some random and non reproductible errors. In particular, some calls to glColor4f raise a GL_INVALID_OPERATION error, although the OpenGL specification says that glColor4f cannot raise an error. When trying to debug with Valgrind (a memory debugger), I find that when calling glEnd, several invalid memory writes occur, which might be responsible for random errors. These invalid writes occur even for small demo, like NeHe OpenGL lesson 2 (I've used the Linux/SDL version of it). I am using Xorg from Mandriva 2006 cooker, i.e. libxorg-x11-static-devel-6.9-1.cvs20051011.2mdk.i586
I can't speek to the particular OpenGL error, but on my laptop version of your card [Mobility FireGL 7800 M7], I got visible errors on the display. It turns out that the video memory was bad. I was able to check using "PC Doctor" under windows on my Thinkpad A31p; I don't know whether there are free diagnostics.
Continuing on my previous comment: After I realized it was a video mem. error, my laptop motherboard got replaced and the errors disappeared.
Closing
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