A possible exception: If the related regionserver was just killed(But HMaster has not perceived that), then we will get a local exception "Connection reset by peer". If this region belongs to a disabling table. what will happen?
ServerShutdownHandler will remove this region from AM#regions. So this region is still existing in RIT. TimeoutMonitor will take care of it after it got timeout. Then invoke unassign again. Since this region has been removed from AM#regions, it will return directly due to the below code:
Then it leads to an end-less loop.