Like you see here in the picture:
http://www.moreawesomethanyou.com/smf/index.php/topic,12828.msg366049.html#msg366049It's not the terrain that is not flat. When I move in a family to the lot when it is zoned as residential, the street is flat. But when I rezone it to apartment, the lot sinks and the blue gaps appear for surrounding lots when I move the family in. The only safe way is to build the lot and never move it. If you move to another spot in the neighborhood, or install the lot from the lot bin, it will sink - but there are exceptions. I did have a basement lot functioning fine moving around or in and out of the lot bin, but when I build a serious apartment bunker for my apocalypse challenge, it always sinks after I move it, whether I move/install it as residential or apartment.
I mostly build basement/bunker lots for the challenge. I don't know if it is because of the basement, or the height of the terrain where you build the lot. These two factors are not conclusive in my testing. So are you having this problem?