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
assert len(scheduler.dagbag.dags.get("test_verify_integrity_if_dag_changed").tasks) == 1
        
        BashOperator(task_id='bash_task_1', dag=dag, bash_command='echo hi')
        SerializedDagModel.write_dag(dag=dag)
        dag_version_2 = SerializedDagModel.get_latest_version_hash(dr.dag_id, session=session)
        assert dag_version_2 != dag_version_1
        scheduled_tis = scheduler._schedule_dag_run(dr, {}, session)
        session.flush()
        assert scheduled_tis == 2
        drs = DagRun.find(dag_id=dag.dag_id, session=session)
        assert len(drs) == 1
        dr = drs[0]
        assert dr.dag_hash == dag_version_2
        assert scheduler.dagbag.dags == {'test_verify_integrity_if_dag_changed': dag}
        assert len(scheduler.dagbag.dags.get("test_verify_integrity_if_dag_changed").tasks) == 2
        tis_count = (
            session.query(func.count(TaskInstance.task_id))
            .filter(
                TaskInstance.dag_id == dr.dag_id,
                TaskInstance.execution_date == dr.execution_date,
                TaskInstance.state == State.SCHEDULED,
            )
            .scalar()
        )
        assert tis_count == 2
        latest_dag_version = SerializedDagModel.get_latest_version_hash(dr.dag_id, session=session)
        assert dr.dag_hash == latest_dag_version
        session.rollback()
        session.close()
    def test_retry_still_in_executor(self):