While building a house I encountered a bug. When building you can make a wall and a floor tile occupy the same tile. This is really annoying because afterwards you cannot clear these tiles anymore.
How to reproduce the bug: Create a floorplan, place walls around it. Drag and create a
new floor from a corner of a wall to into the opposite corner of a wall.
Sorry for the late reply, @leeuw 
The floor was dragged to the top of the column, that’s why it’s overlapping with the wall.
In any case, to build a second floor with auto wall tools you’d have to draw the floor from one column to the opposite one, not inside the house. If you click the undo button does it remove the floor?
I’ve tried to reproduce your screenshot but it doesn’t let me place the floor on the corner of the wall o_o Maybe devs changed something on latest version to avoid this…
I came across this old post of me, and I found out the bug is still there (as of alpha 14). It isn’t possible with floors anymore but it’s still possible with slabs. It’s also possible with free-standing walls and slabs.
How to reproduce:
