context "creating a new tweet" do
    setup do
      login_as :sonnyjitsu
      post :create, :tweet => {:tweet => "This is my tweet"}
    end

    current_user = mock('user')

    should_change("the number of tweets", :from => 0, :to => 1) { Tweet.count }
    should_redirect_to("the tweet page") {tweet_path(Tweet.last)}
  end

The error is this:

sonnyjitsu@ubuntuboy:~/Development/barter140$ rake
(in /home/sonnyjitsu/Development/barter140)
/usr/bin/ruby1.8 -I"lib:test" "/usr/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake/rake_test_loader.rb" "test/unit/helpers/tweets_helper_test.rb" "test/unit/tweet_test.rb"
Loaded suite /usr/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake/rake_test_loader
Started

Finished in 0.000353 seconds.

0 tests, 0 assertions, 0 failures, 0 errors
/usr/bin/ruby1.8 -I"lib:test" "/usr/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake/rake_test_loader.rb" "test/functional/tweets_controller_test.rb"
/usr/lib/ruby/gems/1.8/gems/thoughtbot-shoulda-2.10.2/lib/shoulda/context.rb:398:in `send': undefined method `mock' for TweetsControllerTest:Class (NoMethodError)
	from /usr/lib/ruby/gems/1.8/gems/thoughtbot-shoulda-2.10.2/lib/shoulda/context.rb:398:in `method_missing'
	from ./test/functional/tweets_controller_test.rb:43:in `__bind_1258776323_452708'
	from /usr/lib/ruby/gems/1.8/gems/thoughtbot-shoulda-2.10.2/lib/shoulda/context.rb:293:in `call'
	from /usr/lib/ruby/gems/1.8/gems/thoughtbot-shoulda-2.10.2/lib/shoulda/context.rb:293:in `merge_block'
	from /usr/lib/ruby/gems/1.8/gems/thoughtbot-shoulda-2.10.2/lib/shoulda/context.rb:288:in `initialize'
	from /usr/lib/ruby/gems/1.8/gems/thoughtbot-shoulda-2.10.2/lib/shoulda/context.rb:169:in `new'
	from /usr/lib/ruby/gems/1.8/gems/thoughtbot-shoulda-2.10.2/lib/shoulda/context.rb:169:in `context'
	from ./test/functional/tweets_controller_test.rb:37
	from /usr/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake/rake_test_loader.rb:5:in `load'
	from /usr/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake/rake_test_loader.rb:5
	from /usr/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake/rake_test_loader.rb:5:in `each'
	from /usr/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake/rake_test_loader.rb:5
/usr/bin/ruby1.8 -I"lib:test" "/usr/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake/rake_test_loader.rb"
Errors running test:functionals!


mocha (0.9.8)
thoughtbot-shoulda (2.10.2)
rails 2.3.4



===SOLUTION===
Should run any non-shoulda code (such as mocks and stubs) in the setup block