When I introduce extra clipping planes, I expect OpenGL to reconstruct clipped polygons so that I see a closed clipped polygon. This does not seem to be happening for me on Mac OS X 10.4.7. All I get is the unclosed polygon. With two clipping planes I get just get two part edges. The red book clearly states, “…polygons that get clipped automatically have their edges reconstructed appropriately by OpenGL.” I’m pretty sure it used to be OK with Mesa.
can you post some code and a screenshot? better still you may want to move this and that to the mac forum here. if its a bug you may have company there.