The worst key I ever had the misfortune to drive belonged to the people l worked for.
It was a new (at the time) Renault Megane - the model that looked to be designed by people who didn’t know what the others were doing.
Amongst the bad faults was the fact that if you were driving with the radio on and a traffic alert came through the dashboard shut down until the alert finished, it also had some very odd brakes, and a lot more.
The worst bit was I dropped the car off with a list of issues. The next week I saw the person who I had handed the keys to, the next driver apparently looked at the list and said fine and took it out!