I was playing around with Terragen 3 and noticed something strange happening after the final GI pass is done. It doesn't happen everywhere. Only in some places and in this images it's not so clearly. However, I hope it's enough for figuring out what it is.
The first image is a screen capture of the rendering right before the final GI pass and the second one is the final. Notice the bright and dark line under the red line at the edge of that cliff. As I said, it's not so clear in this image, but often if there are clouds in that area those lines are a lot uglier.
Regards,
Terje