I own two gas stations and there are big problems determining when a gas nozzle is defective or just acting funny. I sell fuel to 1000 cars per day at each one of my stores. One time a nozzle won't shut off for someone and then it will work fine for the next 50 cars or for the next week, who knows. You can't replace a $150 nozzle just because one car had a problem. There are about 10 factors that will make a nozzle not shut off and most of them aren't related to the nozzle. If you have Vapor Recovery in your area then look out.
Those nozzle, hose, breakaway combinations are under-engineered to the max. And stations are forced to use that crap by the EPA or DNR in that area.
How the nozzle senses backpressure is through a small air tube in the spout. If that air tube is restricted by it'* position in car, by a clogged vapor line because the idiot before you overfilled his car on purpose, by gasoline in the vapor line from non functioning venturi tubes in the hose, or etc etc. ... well.... It'* just complicated.
Customers abuse the hell out of the nozzles so employees become indifferent to their complaints. Sorry, its just the way it is.
Take my advice.. stay next to the gas nozzle when the car is nearing full.
Wash your windows after your done fueling.
By the way, gas spills can cost a gas station owner his business. Law suits, fines, injuries etc. Gas station owners do not take them lightly.