Problems on bug#8931

I dont know what testcase can trigger the bug described in the link below:
https://github.com/rails/rails/issues/8931

1) the man's code is not complete. because it lacks the Validation Helpers!
2)this is my code:
class Post < ActiveRecord::Base
  attr_accessible :content, :name, :title

  validates :name,   :presence => true, :on => :create  
  validates :name,   :presence => true, :on => :update, :if => :false
end

unfortunately, such code cant even trigger the bug!  WHY??? so weird!
anyone help?