Has any accident been prevented by bending or breaking the rules?
First I have to admit that I have no idea what FAA regulations said about passengers taking part in controlling an airplane in 1989, but I guess it was at least frowned upon, even if the passenger happened to be a pilot.

The United Airlines flight 232 made a stunningly successful "landing" at Sioux City airport, much due to the fact that they took an extra crew member from the cabin to control the throttles of their severely disabled aircraft.

Unfortunately 112 people lost their lives as the plane cartwheeled after a very rough touchdown, and one might ask what is so successful about that... well: 184 people survived a situation that was in subsequent simulations deemed to be impossible to manage.

The throttle operator was a UA training check airman Dennis Edward Fitch, riding as a passenger on this ill-fated flight. It has been stated in many sources again and again, that without his presence in the cockpit, the outcome would have been much worse, even the worst possible.

It just so happened, that Fitch had been practicing a similar scenario in a simulator after a fatal crash of Japan Airlines flight 123 in 1985. The reason of that crash was total loss of hydraulics, Fitch wanted to find out if an airliner could be controlled with throttles only. Luckily, it was somewhat possible.