What happens if you put leaded petrol in your car instead of unleaded?

What happens if you put leaded petrol in your car instead of unleaded?
Autos made within the past 25 years or so have a fuel filler opening that only permits the insertion of the small unleaded gasoline nozzle, if that is the required fuel for your auto. The larger nozzles of leaded gasoline (still used in airplanes) and diesel will not fit.

Use of leaded gasoline in modern automobiles causes the catalytic converter to malfunction and fouls the spark plugs. This, in turn, alters the operation of the engine in ways that can rapidly lead to damage. Rapidly, in this case, can mean as little as fifteen minutes. Even if the engine is undamaged, it will cost hundreds of dollars to replace the catalytic converter.