Ok, something I haven't noticed before and didn't read about here before so...
As an example;
I have 1 tile left and select a building I want at the bottom of the list. It is placed on the tile and in the que.
The list refreshes itselfs and therefor it selects another building.
I want to buy the building I placed on my last tile, but I don't select it on the map, instead I leave (?) the active selection on the one in the list, click buy and presto 2 (different) buildings on 1 tile, 1 completed the other being build.
This isn't easy (or maybe even impossible) to do if there are more tiles free as normally after placing a building the game selects another free tile
