Paperclip wrong attachment url on validation errors: http://stackoverflow.com/a/5995636/2425581

in_model.rb

class ModelName
  # declare papaerclip :avatar file
  # ...
  after_validation :avatar_reverted?
  
  def avatar_reverted?
    unless self.errors[:avatar_file_size].blank? or self.errors[:avatar_content_type].blank?
      self.avatar.instance_write(:file_name, self.avatar_file_name_was) 
      self.avatar.instance_write(:file_size, self.avatar_file_size_was) 
      self.avatar.instance_write(:content_type, self.avatar_content_type_was)
    end
  end
  # ...
end