I used to own a gas station, and we found that with the nozzles that have the holes around the perimeter of the tip (to suck the fuel vapor out as it is displaced by fuel entering the tank), if a person adds fuel slowly enough, the vapor return would actually suck fuel back to the station's main tank.
We first became aware of this when a customer complained that he added more fuel to his car than it could hold, so I took my Civic over to the same pump and with the customer controlling the flow, we actually put over 25 gallons into that Civic; this was with that Civic's tank 3/4 full before we started.
Needless to say, we had all of our nozzles and pumps recalibrated so that the minimum flow that could be achieved would exceed the point at which the vapor return would suck fuel back to the tank.
Essentially, the problem was caused by sensors that were insufficiently sensitive to fuel coming back up the filler tube of the vehicle, which should trigger the pump to shut off. Our nozzles required a relatively high rate of flow back up the filler tube in order to trigger the shut off.