Why doesn't Mocha complain about expectations?

Question

I'm messing around with Mocha for days and cannot seem to get it working properly.

I'm using Rails 3.1.0.rc4 and Mocha 0.9.12, running under Ruby 1.9.2-p180. In my Gemfile I have a

gem 'mocha', :require => false

and I require Mocha in the last line of my test_helper.rb

require 'mocha'

as suggested in various questions and blog posts.

However, when creating a new Rails project and writing simple tests like the following:

  test "test 1" do
    User.any_instance.expects(:something).returns(true)
    u = User.new
    assert u.something
  end

  test "test 2" do
    User.any_instance.expects(:something).returns(true)
    u = User.new
  end

  test "test 3" do
    u = mock()
    u.expects(:something).at_least_once
  end

All test pass without any errors. So, mocking works (because :something is not a real function) but in the second and third test I would expect Mocha to complain about :something not being called.

Running tests with "MOCHA_OPTIONS=debug" says

Detected MiniTest version: 1.6.0
Monkey patching MiniTest >= v1.4.2 and <= v1.7.2

Can anyone tell me what I'm missing here?