The test fails quite regularly - usually in one of many jobs:

Example


https://github.com/apache/airflow/pull/12850/checks?check_run_id=1506517544

```
______________ TestSchedulerJob.test_scheduler_task_start_date ________________

self = <tests.jobs.test_scheduler_job.TestSchedulerJob testMethod=test_scheduler_task_start_date>

    def test_scheduler_task_start_date(self):
        """
        Test that the scheduler respects task start dates that are different from DAG start dates
        """
    
        dagbag = DagBag(dag_folder=os.path.join(settings.DAGS_FOLDER, "no_dags.py"), include_examples=False)
        dag_id = 'test_task_start_date_scheduling'
        dag = self.dagbag.get_dag(dag_id)
        dag.is_paused_upon_creation = False
        dagbag.bag_dag(dag=dag, root_dag=dag)
    
        # Deactivate other dags in this file so the scheduler doesn't waste time processing them
        other_dag = self.dagbag.get_dag('test_start_date_scheduling')
        other_dag.is_paused_upon_creation = True
        dagbag.bag_dag(dag=other_dag, root_dag=other_dag)
    
        dagbag.sync_to_db()
    
        scheduler = SchedulerJob(executor=self.null_exec, subdir=dag.fileloc, num_runs=2)
        scheduler.run()
    
        session = settings.Session()
        tiq = session.query(TaskInstance).filter(TaskInstance.dag_id == dag_id)
        ti1s = tiq.filter(TaskInstance.task_id == 'dummy1').all()
        ti2s = tiq.filter(TaskInstance.task_id == 'dummy2').all()
        self.assertEqual(len(ti1s), 0)
>       self.assertEqual(len(ti2s), 2)
E       AssertionError: 1 != 2

tests/jobs/test_scheduler_job.py:2415: AssertionError


```

tests/jobs/test_scheduler_job.py
execution_date=DEFAULT_DATE,
            start_date=DEFAULT_DATE,
            session=session,
        )
        ti1a = dr1.get_task_instance(task_id='dummy', session=session)
        ti1a.state = State.SCHEDULED
        ti1b = dr1.get_task_instance(task_id='dummy_b', session=session)
        ti1b.state = State.SUCCESS
        session.commit()
        ti2 = dr2.get_task_instance(task_id='dummy', session=session)
        ti2.state = State.SCHEDULED
        session.commit()
        ti3 = TaskInstance(dag3.get_task('dummy'), DEFAULT_DATE)
        ti3.state = State.SCHEDULED
        session.merge(ti3)
        session.commit()
        dagbag = DagBag("/dev/null", include_examples=False, read_dags_from_db=False)
        dagbag.bag_dag(dag1, root_dag=dag1)
        dagbag.bag_dag(dag2, root_dag=dag2)
        dagbag.bag_dag(dag3, root_dag=dag3)
        dagbag.sync_to_db(session)
        session.commit()
        scheduler = SchedulerJob(num_runs=0)
        scheduler.dagbag.collect_dags_from_db()
        scheduler._change_state_for_tis_without_dagrun(
            old_states=[State.SCHEDULED, State.QUEUED], new_state=State.NONE, session=session
        )
        ti1a = dr1.get_task_instance(task_id='dummy', session=session)
        ti1a.refresh_from_db(session=session)
        assert ti1a.state == State.SCHEDULED