This case is from CRuby's test_rand.rb, test_random_ulong_limited_no_rand:

  def test_random_ulong_limited_no_rand
    c = Class.new do
      undef rand
      def bytes(n)
        "\0"*n
      end
    end
    gen = c.new.extend(Random::Formatter)
    assert_equal(1, [1, 2].sample(random: gen))
  end

The problem here is that the random: passed in still gets used by calling rand which produces a random number, rather than using the bytes method to generate a random number {which should be the same always due to the constant return value).

This broke along with other changes to Random to support the SecureRandom gem and user-defined random byte generation.