I built a building (dining hall) next to my own custom building, and a bit of my dining hall overlapped my already-built custom building. I did not notice this at first, and when I started the construction, my hearthlings would construct everything except the roof and the wall that is overlapped.
Also… for some weird reason, since I tried to destroy the building, it took out some dirt. I tried to fill it with wood, and succeeded, but then I tried to re-build the dining hall. The foundations of the new dining hall overlapped with the foundations of the wood floor that I used to cover up the land, and the hearthlings were successful in building an overlapped foundation, so it made a very weird graphical glitch.