Prevent stubbing nonexistant method with rspec-fire and rspec-mocks. Discussed at http://sebastien.saunier.me/blog/2013/10/06/on-testing.html

_rspec.rb

class God
  #Imagine the welcome person has been removed
  # def welcome_person(person)
  # end
end

class Person
  def initialize(god)
    @god = god
  end

  # Then the API contract from God to Person is broken
  def die
    @god.welcome_person(self)
  end
end

# rspec-mock
describe Person do
  context "Using rspec-mock, when a person die" do
    it "God should welcome him (incorrect test)" do  # GREEN (but should not)
      god = double("god")
      god.stub(:welcome_person)  # rspec-mock does not check if God actually implements it.
      person = Person.new(god)

      god.should_receive(:welcome_person).with(person)
      person.die
    end

    it "God should welcome him (correct test)" do   # RED (as it should)
      god = God.new
      person = Person.new(god)
      god.should_receive(:welcome_person).with(person).and_call_original # Actually calls God implementation
      person.die
    end
  end
end

###################################

require 'rspec/fire'
RSpec.configure do |config|
  config.include(RSpec::Fire)
end

# rspec-fire
describe Person do
  context "Using rspec-fire, when a person die" do
    it "God should welcome him" do  # RED  (as it should)
      god = instance_double "God"
      person = Person.new(god)
      god.should_receive(:welcome_person).with(person)
      person.die
    end
  end
end

Gemfile

source 'https://rubygems.org'
ruby "2.0.0"

gem "rspec"
gem "rspec-mocks"
gem "rspec-fire"

result

$ bundle exec rspec _rspec.rb
.FF

Failures:

  1) Person Using rspec-mock, when a person die God should welcome him (correct test)
     Failure/Error: @god.welcome_person(self)
     NoMethodError:
       undefined method `welcome_person' for #<God:0x007f82d9cba918>
     # ./rspec.rb:14:in `die'
     # ./rspec.rb:34:in `block (3 levels) in <top (required)>'

  2) Person Using rspec-fire, when a person die God should welcome him
     Failure/Error: god.should_receive(:welcome_person).with(person)
       God does not implement:
         welcome_person
     # ./rspec.rb:52:in `block (3 levels) in <top (required)>'

Finished in 0.0014 seconds
3 examples, 2 failures

Failed examples:

rspec ./rspec.rb:30 # Person Using rspec-mock, when a person die God should welcome him (correct test)
rspec ./rspec.rb:49 # Person Using rspec-fire, when a person die God should welcome him