What faults does the middle have?
SELECT fault_short_name from Part_Faults WHERE part_id in  ( SELECT T2.part_id FROM Parts AS T1 JOIN Part_Faults AS T2 ON T1.part_id   =   T2.part_id GROUP BY T1.part_name ORDER BY count ( * )  ASC LIMIT 1 ) 
the middle refers to a part or item that is centrally located or has a median position in sorted order; faults of the middle part are identified by querying the Part_Faults table for faults associated with the part_id of the middle part.