I remember reading an accident report from the US many years ago in which a father was flying with his son who had recently been hospitalised for mental illness. Unknown to the father, the son had stopped taking his medication.
In the cruise, the son suddenly and unexpectedly pulled the mixture control to ICO. The father, somewhat surprised, managed to keep the engine going by restoring the normal state of affairs. Not satisfied with this, the son then reached across, turned off the magnetos, removed the key and threw it out of a window.
The father made a satisfactory dead-stick landing, AFAIR with no injuries sustained by either party.