People put the wrong fuel in their cars all the time.
One reason for this is that sometimes a person will be filling a can with unleaded at the same time as topping up their diesel car. Somehow, the nozzles get put back in the wrong holsters.
Today’s invention is a simple fix, which would also make life hard for mischevious folk who swap nozzles around purely for amusement.
Each nozzle and holster would have male and female features which prevented, say, the diesel nozzle ever being put into the hi-octane holster.