Summary:
Starting a new farm designation outside an existing one and then dragging over the original designation will create an overlapping designation. Whist the many many farm layers create a beautiful golden effect the farm gods were angered and the game crashed before I could see whether crops would grow,
Steps to reproduce:
Place farm designation
Start new farm designation outside original designation, drag mouse over to other corner.
I haven’t encountered this issue since I created another farm plot next to one that wasn’t still finished. It didn’t let me overlap the red areas.
I wonder if the state of the original farm factors in… @Geoffers747, had there been any activity in the original farm before you attempted to overlap?