I noticed that some maps that come with the game have elevations that you can't see on the screen, but do restrict LOS. Rolling over the mouse pointer gives an elevation, but no hillside graphics are shown on the map. The invisible hills are all 'hillside' elevations, that is 5m, 15m, 25m, etc.
I tried to recreate this in the editor. I made a 1 hex level 1 10m hill. All the surrounding hexes automatically went to 5m hillsides as normal. I then 'erased' the hill, that is put level 0 clear terrain in its place. The hillside graphics in the surrounding 6 hexes disappeared, but the rollover message still indicated a level 5 elevation in all of them. The hillsides remained, though the hill had gone. The hillsides had to be erased by hand.
This is not really a problem in the editor if you know about it and can erase the hillsides by hand, but I was a teeny bit surprised to find these invisible hills in the maps that came with the game.
Don't shout at me Don!
Regards Kesh