When you put petrol into a Diesel engine – there are plenty horror stories of how it can totally wreck a cars engine. Imagine this: You are in a hurry, you pull in at a fuel station, ask for Rs. 500 worth of fuel not specifying what you want and then dash off only to realize that the fuel filled was the wrong one. What do you do?
Modern diesel engines are really sensitive to fuel quality and putting petrol in a diesel car can ruin a lot of things quite quickly. Similarly, high precision petrol engines will choke up if fed a diet of diesel. If you realize your mistake soon enough, you can bring things under control.