I have just returned from purchasing fuel at a well known supermarket chain. When I arrived at the pump it was showing the previous customer's total of 42c for 0.27 litres of fuel - an odd amount, which is why I remember it. When I removed the nozzle from the pump the digits did not reset, nor (as I remember) was there the usual "You have selected Diesel" or whatever it is that the automatic voice says in Spanish. I looked towards the cash kiosk for a response but as I didn't get one I left the fuel nozzle in the filler of my car and went over. I asked them if the pump was working and they (two young women) told me it was. They told me to squeeze the trigger, but I said that the numbers were not at zero. They said it was OK so I went back to the car. As the display still showed the same amount as it had when I first arrived I replaced the nozzle in the pump and drove to the one next to it. There, everything worked as normal and I filled the car with fuel to the value of 55,00€. At the kiosk I paid with my credit card. I could not see the amount on the till display but when I checked my receipt I had been charged the additional 42c. I protested, but as my Spanish is not perfect they simply dismissed me, and my wife told me not to make a fuss.
OK, only 42p you may say. But this is not the frst time. Last time (at the same fuel station) I had 60c added to my bill, which I didn't notice at the time I paid. On that occasion the attendant had told me to replace the nozzle and try again. I didn't notice the discrepancy until I got home. If the attendant can get away with this every 10 minutes for an average of 50c then that amounts to a 120€ weekly bonus, based on a 40 hour week. Most people, like me, are not going to complain over such a trivial amount.
Keep your wits about you. Make sure the display shows all zeroes before you start to fill up... and hope you are not going to be charged if they don't reset!