How would you all handle this situation?

Attendants mistake.
You owe them $0.

If I could verify the gas was actually in the tank I'd pay the difference. If I didn't have the money with me I'd pay it the next time. I realize I'm probably not obligated to pay but I would anyway even though it wasn't my fault. I've had people forgive similar mistakes of mine over the years.

That's the thing, is/was the extra gas actually in your gas tank?

If the extra gas was actually in your cars gas tank, personally I would pay for the gas, people are human, they make mistakes. Doesn't sound like the gas attendant was rude, after reading OP.

BTW, where do you get gas that has a gas attendant, do you mean the cashier, or was it actually full service gas station? I'm asking because you would know If you put $40 worth of gas in your tank If you personally pumped the gas.

Once back in 2000 I was driving up around Ashville, NC (US), I was driving a new full size Dodge pickup truck (gas hog). I stopped at this small but very busy gas station off the interstate, keep in mind this is back when you didn't have to pay for the gas up front (cash). I pumped $40 worth of gas, went in to pay at the register, the lady asked If I pumped $20 worth of gas, I said no, I pumped $40, she said no, you pumped $20. I didn't have time to debate the subject & the lady was very nice, I laid $40 on the counter at the cash register & walked out.

Before I got in the truck to leave I double checked the gas pump to see If I did in fact pump $40 worth of gas, sure enough, exactly $40. I just smiled & drove off.

Thanks for all of your posts. All of your responses covered the range of emotions and responses I had to this situation.

I realized that I should have given a little more detail instead of just the facts of what happened, because there were more factors that played into my decision:

This is a local, full service gas station in my neighborhood. I'm very friendly with the owner and we always have a nice conversation whenever I'm there. This new attendant he just hired spoke very broken English and it was very clear to me that he is new to this country and perhaps not familiar with how American business culture works. Some of you might say that this is all irrelevant, and you may be right, but this was just my take on things. At any rate, after he made the mistake, I could see his face really drop and almost turn white. He got a bit rude with me and said, "You'll pay this the next time you're here, yes?" I replied by saying, "Why should I pay? You made the mistake." The attendant then looked at me and said, "Get out of here." The owner then came over and apologized. I basically just laughed and looked at the attendant and said, "Ha, you're telling me to get out of here because you made a mistake?" and I drove away. I could see the owner was embarrassed as he apologized and tried to smile.

As I drove away, I thought of a few different things: First, I thought about how I'm friends with the owner and he is really just a nice, hard working business owner. I then thought about how difficult life must be for this gas station attendant who probably has a very hard time understanding and adjusting to how things work in this country. I then also thought about the fact that the extra $20 of gas that he put in my gas tank was probably going to come out of his pay and that would probably be about a half a day's pay for this man.

So, while I felt that I didn't have the obligation to pay, I felt that a better lesson could be taught by paying, so I drove over the bank, took out the money, and drove back to the gas station. As I pulled into the gas station, I drove over to where the owner was and he smiled, kind of in an embarrassed way. I gave him the extra $20 and he gave me the change since the extra gas was a little short of another $20. I just looked at him and said, "You know, I'm doing this because you're a nice guy and I like you, but that really wasn't right of him to tell me to get out of here because he made the mistake." The owner just shook his head and said, "I know. I'm sorry."

As I drove away, I wondered if I really did the right thing to drive back with the money, or if a better lesson could have been taught to this gas station attendant if the money had in fact been taken out of his pay. I then just basically thought about how the owner will likely speak to him, tell him the way business works in this country, and maybe enough of a lesson is learned in that sense. I also now know that the next time I go to the gas station, the owner and I can still have our nice conversations, without the uncomfortable feeling looming over us, due to the weird situation that took place. After all, I would be using the gas, and the next time I would have needed gas, I would be going back to his station anyway. So, I felt like it was one of those situations where I didn't have the obligation to pay, but I felt like more harm than good would have come out of not paying, for the gas that was now in my gas tank anyway. At least, that's just how I saw it.

Look at it like this, now each time you go back to that same gas station to buy gas, you can always joke with the owner & ask how much free gas they're giving away today?

Buy a gallon, get a free gallon?

I believe in the golden rule. One look at my gas gauge would tell me if he put in the extra. If I thought he did, I'd pay the next time I was in the area. I'm also the kind of person that tells the attendant if they undercharge me, or give money back if they give me too much change.

I would have paid too, even if I had to swing back by. People make mistakes. I would, though, have to see the pump to verify that the extra actually got put in there - I have 50+ gallon tank on my pickup so $20 worth of gas won't even move the needle.