I have some experience with this problem. Whenever I want to construct hills on the edge of the map, I use the biggest brush and put the cursor right on the edge of the map when I elevate (alt-d activated). This way the land is elevated even beyond the border of the map (presumably as it is invisible), and for me this has prevented the occurrence of these black and green error tiles you mentioned (at least, up until 5 levels of elevation, I have not tried anything higher). Decreasing the elevation is done in the same way. Now my suggestion would be to to start increasing the elevation on the same spot again, using the method described above, until you reach the height of your original hill. Next, decrease the elevation in the same manner. If this does not do the trick, you might consider flattening the elevation (one of the options in the top left corner of the editor).