In Build mode on any lot, try to create a wall on the tiles directly next to the sidewalk.
The game will not allow this, and state that the location is out of bounds. If you try to 
build a wall diagonally in this area however, even though the game will state that the 
location is out of bounds, it will allow you to build it. The wall will be stuck here 
and the game will not let you delete it. The only way to remove it is to use the 
"move_objects" on code.