UsersController#create:

  def create
    @user = User.new(params[:user])
    if @user.save
      flash[:notice] = "Account registered!"
      redirect_back_or_default account_url
    else
      render :action => :new
    end
  end

UsersControllerTest:

  context "on POST to :create" do
    setup do
      controller.stubs(:require_no_user).returns(true)
      @the_user = User.generate!
      User.stubs(:new).returns(@the_user)
      post :create, :user => { :login => "bobby", :password => "bobby", :password_confirmation => "bobby" }
    end

    should_assign_to(:user) { @the_user }

    context "with successful creation" do
      setup do
        @the_user.stubs(:save).returns(true)
      end

      should_respond_with :redirect
      should_set_the_flash_to "Account registered!"
      should_redirect_to("the user's account") { account_url }
    end

    context "with failed creation" do
      setup do
        @the_user.stubs(:save).returns(false)
      end

      should_respond_with :success
      should_not_set_the_flash
      should_render_template :new
    end
  end

Results:

1) Failure:
test: on POST to :create with failed creation should not set the flash. (UsersControllerTest)
method assert_rejects in assertions.rb at line 56
method __bind_1246115268_943483 in macros.rb at line 43
method call in context.rb at line 253
method test: on POST to :create with failed creation should not set the flash. in context.rb at line 253
Did not expect the flash to be set, but was {:notice=>"Account registered!"}

  2) Failure:
test: on POST to :create with failed creation should render template :new. (UsersControllerTest)
method __bind_1246115268_974053 in macros.rb at line 193
method call in context.rb at line 253
method test: on POST to :create with failed creation should render template :new. in context.rb at line 253
expecting <"new"> but rendering with <"">

  3) Failure:
test: on POST to :create with failed creation should respond with success. (UsersControllerTest)
method assert_accepts in assertions.rb at line 50
method __bind_1246115269_76190 in macros.rb at line 133
method call in context.rb at line 253
method test: on POST to :create with failed creation should respond with success. in context.rb at line 253
Expected response to be a 200, but was 302