Major Description
When the server crashes, it's SCP checks if there is already a procedure assigning the region on this crashed server. If we found one, SCP will just interrupt the already running AssignProcedure by calling remoteCallFailed which internally just changes the region node state to OFFLINE and send the procedure back with transition queue state for assignment with a new plan.
But, due to the race condition between the calling of the remoteCallFailed and current state of the already running assign procedure(REGION_TRANSITION_FINISH: where the region is already opened), it is possible that assign procedure goes ahead in updating the regionStateNode to OPEN on a crashed server.
As SCP had already skipped this region for assignment as it was relying on existing assign procedure to do the right thing, this whole confusion leads region to a not accessible state.