My code is the same as shown on http://www.salesforce.com/docs/developer/cookbook/Content/apex_controlling_recursive_triggers.htm with the following differences:

trigger AutoCreateFollowUpTasks on Task (before insert) {

    // Before cloning and inserting the follow-up tasks,  
    // make sure the current trigger context isn't operating  
    // on a set of cloned follow-up tasks.  
    
    if (!FollowUpTaskHelper.hasAlreadyCreatedFollowUpTasks()) {

        List<Task> followUpTasks = new List<Task>();
        Integer taskCount = 0;
        
        for (Task t : Trigger.new) {

            // Fake an arbitrary business rule failure

            if (taskCount++ == 1)
            {
                t.addError('Aribitrary error');
            }
            else if (t.Create_Follow_Up_Task__c) {

                // False indicates that the ID should NOT  
                // be preserved  
    
                Task followUpTask = t.clone(false);
                System.assertEquals(null, followUpTask.id);

                followUpTask.subject = 
                FollowUpTaskHelper.getFollowUpSubject(followUpTask.subject);
                if (followUpTask.ActivityDate != null) {
                    followUpTask.ActivityDate =
                      followUpTask.ActivityDate + 1; //The day after  

                }
                followUpTasks.add(followUpTask);
            }
        }
        FollowUpTaskHelper.setAlreadyCreatedFollowUpTasks();
        insert followUpTasks;
    }
}

    static testMethod void testCreateFollowUpTasks() {
        List<Task> tasksToCreate = new List<Task>();
        for (Integer i = 0; i < NUMBER_TO_CREATE; i++) {
            Task newTask = new Task(subject = UNIQUE_SUBJECT,
                    ActivityDate = System.today(),
                    Create_Follow_Up_Task__c = true );
            System.assert(newTask.Create_Follow_Up_Task__c);
            tasksToCreate.add(newTask);
        }

        Database.insert(tasksToCreate, false);
        System.assertEquals(3,
                            [select count()
                             from Task
                             where subject = :UNIQUE_SUBJECT
                             and ActivityDate = :System.today()]);

        // Make sure there are follow-up tasks created  
    
        System.assertEquals(3,
           [select count()
            from Task
            where subject = 
           :FollowUpTaskHelper.getFollowUpSubject(UNIQUE_SUBJECT)
           and ActivityDate = :System.today()+1]);
    }