rspec.rb

describe "some group" do
  example "example 1" do
    expect(@triangle).to have(2).sides
  end
  
  describe "some sub group" do
    it "has another example, using the 'it' alias" do
      expect(@circle).to be_square
    end
  end
  
  example "example 2" do
    expect(@square).to have(3).sides
  end
end

test_pollution.rb

require 'minitest/autorun'

class User
  def self.login(username, password)
    if username == 'fred' && password == '123'
      @logged_in = true
    end
  end

  def self.logged_in?
    @logged_in == true
  end
end

class UserTest < MiniTest::Unit::TestCase
  def test_logs_in_if_correct_credentials_used
    User.login('fred', '123')
    assert User.logged_in?
  end

  def test_doesnt_log_in_if_incorrect_credentials_used
    User.login('bill', '321')
    refute User.logged_in?, "User was logged in, but expected not to be"
  end
end