const.rb

module A
  THING = 1
  module B
    def self.thing
      puts THING
    end
  end
end

module X
  THING = 2
end

module X::Y
  def self.thing
    puts THING
  end
end

puts A::B.thing #=> 1
puts X::Y.thing #=> const.rb:16:in `thing': uninitialized constant X::Y::THING (NameError)

const2.rb

module A
  THING = 1
  module B
    def self.thing
      puts THING
    end
  end
  module C
    def self.bar
      puts B.thing
    end
  end
end

module X
  THING = 2
end

module X::Y
  def self.thing
    puts THING
  end
end

module X::Z
  def self.bar
    puts Y.thing
  end
end

puts A::C.bar #=> 1
puts X::Z.bar #=> const.rb:27:in `bar': uninitialized constant X::Z::Y (NameError)