Here's three things that can happen:
The previous car filled up and got 300 pesos worth of gas. He pays and drives off. You then pull up and tell the attendant that you would like to fill it up. The attendant starts the pump without resetting the gas pump. He then charges you the amount of the previous customer plus the gas that you got. You just got fleeced and there is no way for you to complain because you don't have proof.
You get gas and you pay the attendant 500 pesos. He has 50 pesos in his other hand and he tells you that you still owe him because you only paid him 50 pesos. You just got fleeced!
You give the attendant your credit card. The attendant has a hand held credit card machine and underneath it he has a credit card "reader". He swipes your credit card twice. Once for the credit card machine and another swipe in the credit card reader. The credit card reader stores your credit card numbers and it is used to make fraudulent transactions later.
I always recommend being aware of your surroundings. If a business takes credit cards, I would always pay with a credit card instead of cash. Always save your receipts until after you have received your statements from your credit card company. You can always dispute unknown transactions with a credit card company. I never use debit cards while traveling. If you were to use a debit card and they swipe it with a card reader, your acc
ount could be drained instantly! Make sure balances of receipts match the amounts on your statements. Are you driving a rental car while on vacation? Rental cars stand out because their license plates begin with "AVB". These rental cars are easy to spot by the gas attendants and you are more prone to being "fleeced". Beware!