It can be easily reproduced (check the image), but it seems to be a false alarm... just realized that while it is obvious for me where the cap should be added - it's not obvious for the renderer.
So not sure if that can be solved. What do you think?
The problem is that in this particular situation I'm working with a ~10M polys terrain model, adding thickness to it isn't that good option for me.
Addition: actually no, it's not a false alarm and should be solved - in the case of single-sided geometry cap should be created at the backside based on normal orientation.