25 Years ago, I flew a KingAir C90 that would trim nose up in the cruise and then, disconnect the autopilot with a violent nose up response after an hour of flight or so. Months went by with this snag, changing autopilots and "Please Report Back". GTFS! Ground Tested, Found Satisfactory was the usual response.
Then one day while hand flying the departure, the trim cable snapped in flight. I reduced speed to find the trim speed and fly it from there on wards to landing.
On the ground the authorities got involved, rightly so, following up on each snag in the Flight Folio, and all those about the autopilot problem.
The trim cable became frayed and was rolling onto the autopilot drive pulley and wasn't able to leave the drive pulley as it had wound up on itself. Hence each flight, it got a little worse and worse and eventually snap.
Lessons learn't back then already, that anything wrong with trims, flight controls, stickiness in full flight control check, go straight back to the hangar!