Cough, splutter...

Seems more and more people are forgetting and putting petrol in their diesel cars with serious consequences. If memory serves, when unleaded petrol first came in, and it was important not to put leaded petrol into cars made to run on unleaded, they solved the problem by making the nozzles a different shape, so you physically couldn't put the wrong sort of petrol in. Surely the same idea could be used for diesel?