Pump nozzles are in use every day by millions of people worldwide and so many of us sometimes make a mistake of putting gasoline in a diesel car or diesel in a gasoline car. We were on a walk the other day and saw a line of cars from a local company that had a sticker on each filler flap describing the type of fuel that goes into the tank. Probably it was a maintenance guy who put them on after rescuing his coworkers a few times when they mistakenly filled the tank with the wrong type of fuel.
It reminded me of my own experience. It was 9PM, December, cold and very dark outside. That week I was driving my mum's car which uses gasoline and I filled up her tank a few days before. But that day my wife gave me a lift and she came to pick me up. It was after 12 hours at work and we stopped at the grocery store on a way home adding an extra hour on top of 12. After the store we stopped at the gas station as we were running out of fuel. I didn't even think about which nozzle I grabbed and put in a filler neck but I realized it was not the right one when I pulled it out. I filled the tank of our diesel car with a gasoline.
It cost me 100€ and at the end I had 50 liters of gasoline in 5 10l containers. I know it is not life threatening if a gasoline is put in a diesel car. I know that the engine can't break if it happens. But it's annoying. I had to park a car, call someone to pick us up, waited there for half an hour in the cold and came home one hour later without a car. I had to borrow the car the next day, go to a garage, wait for them to empty the tank, pay for it, get my wife from work, drive her to the garage so we could drive home both cars. It took me the whole working day to get it sorted.
There is at least the convention of green nozzle for gasoline and a black nozzle for diesel (in Europe at least, because in USA green nozzles are used for diesel). But is it enough? Some can argue that it works with traffic lights (red for stop and green for go) but some people drive trough red light not even noticing their mistake (I was with a friend once in a car when she did it). No wonder then, that people mistakenly fill the gasoline in diesel cars if there is no danger associated with it.
But while there's nothing much to be done on streets (forget cameras and fines), making mistakes on gas station could be simply prevented. For example: round nozzle for gasoline, square for diesel, triangular for natural gas ... each of different colour. Filler necks should be of the shape to accept only appropriate nozzle. That simple.
Edit: I just remembered a series of ads from Slovenian Ministry of internal affairs inviting drivers to drive carefully, within speed limits, fastening seat belts, etc. A part of every ad were these smiling faces that resemble my shaped nozzles :). But without colours ...