Every fuel station I have ever been in around here has 4 buttons on the pump. the customer must push one of those buttons to get fuel.
One of them is Diesel.
the other three are 87, 89, and 91 octane gasoline.
In order to get gasoline, you must pick up the gasoline nozzle, then push one of the gasoline buttons.
The same goes for diesel, of course. You must pick up the diesel nozzle, then push the Diesel button.
I one grabs the Diesel nozzle and pushes one of the gas buttons, there will be no fuel flow, and vice versa.
Seems to me the only way to make a mistake around here would be to really try!