I imagine this is due to how ActiveSupport::Concern handles dependency resolution? It's pretty surprising though because you can use most other Rails class methods (before_filter, helper_method, belongs_to, etc. etc.) in the included method's block.

concern.rb

module Surprisable
  extend ActiveSupport::Concern

  included do
    attr_accessor :this_aint_gonna_work
  end
end

module Wtfable
  extend ActiveSupport::Concern

  module ClassMethods
    attr_accessor :neato
  end
end

class WttfClass
  include Surprisable
  include Wtfable
end

WtfClass.this_aint_gonna_work = 'hey'
#=> NoMethodError: undefined method `this_aint_gonna_work` for WtfClass:Class

WtfClass.neato = true
#=> true