Partially offscreen polygons are rendered incorrectly. Visible part of them is
misplaced and texture coordinates are sometimes set to random values. Problem
appears only when clipping to viewport, polygons that are fully located inside
the viewport are rendered correctly.

Here's what glxinfo says about renderer:

OpenGL vendor string: Gareth Hughes, Leif Delgass, José Fonseca
OpenGL renderer string: Mesa DRI Mach64 [Rage Pro] 20051019 AGP 2x
OpenGL version string: 1.2 Mesa 6.4.2

I'm not the sole "victim", the same problem with Mach64 driver (on another card)
is described by another user here (you can get some screenshots there):
http://www.linuxquestions.org/questions/showthread.php?p=2368337

I'm attaching code that reproduces the problem with clipping.

---------------------8< cut here 8<----------------------------------

#include <GL/glut.h>

void display(void)
{
        glClearColor( 0.0, 0.0, 1.0, 0.0 );
        glClear( GL_COLOR_BUFFER_BIT );

        /* The white triangle is located inside the viewport
         and is rendered correctly */
        glColor3f( 1.0f, 1.0f, 1.0f );
        glBegin( GL_TRIANGLES );

        glVertex2f( 0.25f, 0.25f );
        glVertex2f( 0.75f, 0.25f );
        glVertex2f( 0.5f, 0.85f );

        glEnd();

        /* The red triangle is partially offscreen and various clipping
         artefacts occur during render.
           When rendered correctly, the red triangle should look the
         same as white one, just translated to the left */
        glColor3f( 1.0f, 0.0f, 0.0f );
        glBegin( GL_TRIANGLES );

        glVertex2f( -0.15f, 0.25f );
        glVertex2f( 0.35f, 0.25f );
        glVertex2f( 0.1f, 0.85f );

        glEnd();

        glutSwapBuffers();
};

int main(int argc, char **argv)
{
        glutInit( &argc, argv );
        glutInitDisplayMode( GLUT_DOUBLE | GLUT_RGBA );
        glutInitWindowSize( 640, 480 );
        glutCreateWindow( "ATI Mach64 driver bug?" );

        gluOrtho2D( 0, 1, 0, 1 );

        glutDisplayFunc( display );
        glutMainLoop();

        return 0;
}