When working as an engineer on B737-200 in the Gulf I once had an aircraft that failed the Flight Test by dropping the right wing well before the stall. It was straight off a D check and all the controls had been rigged to the book. We adjusted the flap drive shaft by half a turn to lower the right flaps compared to the left. It was indiscernable by looking at them, but cured the problem. I was amazed how a tiny variation of the flap rigging could make such a difference.