Separate servicing of engines on ETOPS aircraft
It would be an ideal ETOPS requirement to have 2 different sets of maintenance crew service the two engines separately using two different batches of engine oil and other stuff. Then they should cross-check each other's work. Well this ideal arrangement NEVER happens in the real world despite several incidents.
One incident that come to mind; some years ago an MAS B777 on an ETOPS flight suffered low oil quantity and pressure on BOTH engines about 30 minutes after takeoff. Luckily they beat a hasty return for landing ( without shutting down any engines due to LOP! )...the maintenance crew had left both engine oil tank caps open after oil servicing! It was a very close call.