In my application Im using offscreen rendering in a Pbuffer for rendering images files of the scene. Reading the Pbuffer content (using glReadPixels()) I get some invalid colored pixels near the upper left and the lower right corner of the rectangle. But this effect occurres only for selected viewport dimensions (e.g. 720x576 and 100x100 pixels is ok, but 400x230 pixels will fail. NVidia GeforceFX5900 used.