I have had two fueling errors to deal with;
1. At TLH, when I went to preflight, one tank was under 1/2 full. The FBO first tried to say that the fuel drain was leaking and that it was my problem, but I asked that he come out and see just how low the tank was and how minimal any "leaking" was (there was evidence of a couple of drips, maybe). Once he looked and saw just how low the tank was, he said, "Hmmm," and waved the truck over with his apologies. They give excellent service at Flightline.
2. At a large airport in the American south, I parked a plane with a large FBO on an extremely busy weekend, with a top-off order. When I came to fly home, they could not find a fuel ticket, and declined to collect a ramp fee (I did not really need the fuel, was buying it as a matter of form). Then, when I got to the plane, it was full-to-the-brim with sparkling blue go-juice. So when I went back to the counter to pay for it, the counter girl (who was overrun with jet-drivers and millionaires) told me, "Look, don't worry about it, I have no idea how much gas we gave you and we've inconvenienced you enough already." She declined a tip, too, but I did tip the line guy who gave me a cart ride back to the plane.
About a year and a half later, the plane's owner called me and told me that the FBO's business office had sent him a bill (they had sold-out to another outfit, and were closing out old accounts). I said it was my hickey, but (of course) I'd just submit the invoice to the club, so it was net-net anyway. He told 'em to pound sand, and they went away.