I had a German car in the 70s that had a terrible stall and rough low speed problem that could not be found. Turned out to be a carb that had a Venturi suspended in the main intake throat that was loose and wiggled at certain idle speeds. Most of the time you could have the air intake off and look right at it and could not see it. The dealer couldn’t find it. I finaly saw it myself.
I had an Opel GT that had an odd miss at 55 mph. It was fine everywhere else, but seeing as how the speed limit was 55 it was an annoyance. Never did fix it. I loved the car other than that.
I only got rid of it because it was made effectively obsolete with the elimination of leaded premium, and I wasn’t about to throw some stupid additive in every time I filled the tank.