It can also not do this too. If you take a look at thread link in my previous comment ( just above yours ) you can see the wbs double back on themselves, to take out a wall with absolutely no buildings inside it ( there was, but they were destroyed by the time the wbs went for that particular wall.
It's rare that this happens, not unlike pekkas movement we have all grown to love/hate. Little "blips" happen, also explained why in my previous post. :)

