callbacks.rb needs active_support/core_ext/object/metaclass because it uses metaclass

activesupport/lib/active_support/callbacks.rb

 require 'active_support/core_ext/array/wrap'
 require 'active_support/core_ext/class/inheritable_attributes'
 require 'active_support/core_ext/kernel/reporting'
+require 'active_support/core_ext/object/metaclass'
 
 module ActiveSupport
   # Callbacks are hooks into the lifecycle of an object that allow you to trigger logic