Here are the steps to reproduce: - Create an application with 2 models: ``` shell rails new assoc_test_app cd assoc_test_app rails generate model Person name:string rails generate model Member person_id:integer rake db:migrate ``` - Edit the models: ``` ruby class Member < ActiveRecord::Base belongs_to :person, :inverse_of => :member accepts_nested_attributes_for :person end class Person < ActiveRecord::Base has_one :member end ``` - Start console (`rails c`) and do: ``` ruby m = Member.new m.build_person(:name => 'John2') m.person.save ``` This gives "SystemStackError: stack level too deep".