This happened to me a loonnnng time ago, but every time I go to the gas station I think about how there should be great big warning signs at every pump.
When you fill up your gas tank and swipe your credit card, have you ever noticed that sometimes it waits until you've finished pumping gas to ask if you want a receipt?
UNTIL YOU CLICK YES OR NO, YOUR TRANSACTION IS NOT FINISHED. EVEN IF YOU HAVE REPLACED THE PUMP.
That means that if you DON'T click yes or no, and then you drive off, the next person to pull up to that pump CAN FILL UP THEIR TANK ON YOUR CREDIT CARD. Because the transaction is still technically open!
And then you'll notice a $120 charge at a Chevron on your credit card statement and you'll realize that you just bought someone a tank of gas.
And you may desperately try to rationalize that you just purchased a buttload of good karma, but I bet you never drive away from another gas pump until the little screen says, "Von's Card Reward Member? Yes or No."