fake_github.rb

require 'sinatra/base'

class FakeGitHub < Sinatra::Base
  def simulate_error!
    @@simulate_error = true
  end
  
  def reset!
    @@simulate_error = false
  end
  
  def simulate_error?
    @@simulate_error
  end
  
  before_do
    if simulate_error?
      halt 403, { errors: { first_name: "can't be blank" } }.to_json
    end
  end
  
  get '/repos/:organization/:project/contributors' do
    json_response 200, 'contributors.json'
  end
  
  get '/repos/:organization/:project/issues' do
    json_response 200, 'issues.json'
  end

  private

  def json_response(response_code, file_name)
    content_type :json
    status response_code
    File.open(File.dirname(__FILE__) + '/fixtures/' + file_name, 'rb').read
  end
end

some_spec.rb

feature 'External request' do
  scenario 'GitHub service failure' do
    FakeGithub.simulate_error!
    uri = URI('https://api.github.com/repos/thoughtbot/factory_girl/contributors')

    response = JSON.load(Net::HTTP.get(uri))

    # some assertions
  end
end