What's the fault description for that?
SELECT T1.fault_description FROM Fault_Log AS T1 JOIN Engineer_Visits AS T2 ON T1.fault_log_entry_id   =   T2.fault_log_entry_id GROUP BY T1.fault_log_entry_id ORDER BY count ( * )  DESC LIMIT 1
fault description for the most frequently visited fault_log_entry; grouping by fault_log_entry_id and ordering by the count of visits to determine the most frequent;