If public.default is the name of a layer, but it is not set as the default layer, the UFO will not save, and pops up with "The document [file-name] could not be saved. The file isn’t in the correct format."
Traceback (most recent call last):
File "lib/doodleDocument.pyc", line 186, in writeSafelyToURL_ofType_forSaveOperation_error_
File "lib/fontObjects/doodleFont.pyc", line 224, in save
File "/Applications/RoboFont_3-5b2.app/Contents/Resources/lib/python3.7/defcon/objects/font.py", line 823, in save
Weird edge case, but took me a while to figure out when I ran into it. The UFO was a generated instance, and I had manually returned it to the foreground (default), background layer naming scheme, so it could work with some of my inelegant scripts.