A320 vibration
Very slight but persistent airframe vibration in flight over 6 months. Engineers formed the opinion that it couldn’t be all that serious. Flight control computers were swapped with other aircraft but the vibration did not transfer. Finally a crew decided that the vibration could no longer be ignored and after 10,000 ft the aircraft returned to departure point. The next day same crew scheduled to fly the same aircraft with identical return.
The horizontal stabilizer shaft runs through bearings that are held in a housing by 4 bolts. One bolt was broken and 2 were loose. This means that the load that was previously taken by 4 bolts is now taken by one with added vibration. Eventual failure of the remaining tight bolt would seem inevitable at which point there are likely to be severe control difficulties or worse.
The moral is be very vigilant and consistent in the application of your own standards. If your well paid expat contract would be jeopardized, don’t sweat it.