There was a rogue Wellington bomber during the war that was pulled off ops due to its tendency to fly around in circles, after intensive investigation they found a section of geodetric was missing from a stabiliser resulting in it flexing up and causing the problems.
I know of a lowly Cessna 152 that would yaw badly in flight, but cure itself at lower speeds and it took ages to find out the reason, it had the later wing mounted landing lights and at cruise the perspex leading edge cover was vibrating across and opening the internal wing up to the airflow, as it decreased speed it vibrated shut and was like that on the ground. Took the poor engineer ages to find it, and he only did when after one flight it didn't fully shut.
..
Last edited by NutLoose; 15th Oct 2015 at 12:11.