A cabin pressurization switch left in the manual position during pre-flight inspections of Helios Flight 522
caused the entire plane to pass out during flight before slamming into the ground killing everyone.
An F16 fighter jet even flew alongside the aircraft when they couldn't get a response from the aircraft and reported a plane full of unresponsive passengers except for one soul in the cockpit behind the controls. This person was Andreas Prodromou, a flight attendant with the airline who was attempting to safely land the plane. Unfortunately he too succumbed to the lack of oxygen and passed out shortly before the aircraft crashed.