Found a bug in RSpec.

false-positive.rb

#RSpec 1.3.0, #Ruby 1.9.1
    describe "Simple class" do
      before :each do
        @c = Class.new do
          attr_accessor :apple
        end.new
        
        @c.apple = []
      end

      it "should change values" do
        # Passes, but it's a false positive:
        #   `apple` is `[]`, not `nil`
        ->{ @c.apple = 100 }.should change(@c, :apple).from(nil).to(100)
      end
    end