Class variable defined on parent class level being set on the parent class level. Looks like a Ruby-powered "feature" of STI.

inheritance_modules_callbacks.rb

# seems to work properly, i.e. exporting_message callback does not pollute
require 'active_support/callbacks'

module Exportage
  def self.included(base)
    base.class_eval do
      set_callback :save, :before, :exporting_message
      def exporting_message
        puts "exporting..."
      end
    end
  end
end

class Record
  include ActiveSupport::Callbacks
  define_callbacks :save

  def save
   run_callbacks :save do
     puts "- save"
   end
  end
end

class PersonRecord < Record
  set_callback :save, :before, :saving_message
  def saving_message
   puts "saving..."
  end

  set_callback :save, :after do |object|
   puts "saved"
  end
end

class ProductRecord < Record
  include Exportage
end

class CarRecord < Record
end

#record = Record.new
#record.save

#person = PersonRecord.new
#person.save

#car = CarRecord.new
#car.save

#product = ProductRecord.new
#product.save

inheritance_modules_cvars.rb

# Class variable being set on the "parent class" level

module Exportage
  def self.included(base)
    base.class_eval do
      after_save 'export_on_deferred_payment'
    end
  end
end

class BaseOrder
  @@callbacks = []
  def self.after_save(something)
    @@callbacks << something
  end

  def self.print_callbacks
    puts @@callbacks.inspect
  end
end

class Order < BaseOrder
  include Exportage
end

class EbayOrder < BaseOrder
end

# BaseOrder.print_callbacks
# >> ["export_on_deferred_payment"]

# Order.print_callbacks
# >> ["export_on_deferred_payment"]

# EbayOrder.print_callbacks
# >> ["export_on_deferred_payment"]

AS::Callbacks uses a class instance variable to save the callbacks, whereas you use a class variable!