It appears that if you attempt to delete a field that had a list or either radio buttons and check boxes, it returns a "form failed to save error". This happens even if you delete all the list elements.

Strangely, the error message itself is in error in that other fields that were changed on the form saved notwithstanding the error message.

The workaround is to change the type of field to "Hidden". Then your form saves and the user doesn't see it.