gistfile1.rb

class A           # top-level A
end

module M
  module ClassMethods
    def create_an_a
      A.new        # top-level A
    end
  end

  module InstanceMethods
    def do_something
    end
  end
end

class B
  class A                    # nested A
    extend M::ClassMethods
    include M::InstanceMethods
  end
  
  def go
    an_a = A.create_an_a     # Calling a method on *nested* A that
    an_a.do_something        # happens to return an instance of
  end                        # *top-level* A. 
end

B.new.go              # unknown method do_something, because the A object you're
                      # calling it on is an instance of top-level A,
                      # not an instance of nested A, and nested A is
                      # the one with the module included.