I earned flying money in high school from pumping gas and washing airplanes at a little FBO.
I forget what the useable fuel is in a C-150, but I once pumped 1.2 gallons more than that into a guy’s airplane after he landed from a half hour flight. Taxied in on fumes, I guess.
Once I was writing out a fuel receipt when I saw this guy’s eyes get huge, and he ran out the door. Turns out he forgot to set the parking brake on his Baron, and it was rolling across the ramp towards the ditch. He managed to get aboard, and prevent further embarassment.
Another time, I climbed up on a C-172 to gas it up, and found both the fuel tank caps hanging on by their safety chains. The guy had flown 3 hours with open fuel tanks.
The worst case of (non-injury) abuse I saw was a guy who had a V35 Bonanza (beautiful airplanes if they’re taken care of). He treated it like a rented mule. Did his own mantainence (he was no mechanic), kept no log books, and annual inspections? who need’s em? You didn’t feel safe even standing next to it. He was a real criminal (ended up in prision, last I knew). The Sheriff came out one day and locked a log chain around the prop, and auctioned it off on the spot to cover some debts. Only salvagers were bidding.