A reader expressed a concern about our taking avgas from a small facility that does not sell a lot of avgas in a month. The risk of contaminated or otherwise bad fuel is probably greater in those cases, and we do take some precautions when using it.
Whenever possible we fuel the night before and then check the fuel sumps in the morning, giving time for water and particulate matter to settle to the bottom of the tank where we can see it. During this operation I have never found appreciable water in the fuel system, and never found any that didn't correlate with precipitation during fuelling. I too have put my credit card slip in a safe that opened with the comnbination that was written on the instruction sheet that I found inside the door that opened with the emergency code, but based on paperwork by the fuel tanks and how often we happen to see people doing official testing-like activities at the holding tank sump, I believe that the neglected airport fuel tank is not that common.
My company has had one instance of a problem from taking on contaminated fuel, and it was from a fairly large operator in the United States. The pilot was ferrying across the country and refilled four empty tanks plus topped up the two half-full inboards. Run up and take off was on the inboard tanks (there's a brief test of the outboards on the checklist, but it's only enough to confirm fuel flow, not really quality). After levelling off at cruise, the pilot switched to the outboard tanks and the engines ran roughly. They ran better on the original inboards, and after return to the airport, it was determined by maintenance that the new fuel load was contaminated with diesel, i.e. probably jet fuel. The FBO insisted that no one else had had a problem, but likely the other piston airplanes that had taken fuel had added the contaminated fuel to already half full tanks. We saw that that level of dilution did not cause an obvious problem on our own inboard tanks. It was when the engines ran off tanks that were filled from empty with the contaminated fuel that the problem occurred.
A few years back, Australia had much of general aviation grounded for a few days because of a contamination problem that originated right at the refinery. A few airplanes had engine problems and there were a couple of helicopter crashes attributed to the problem. Ironically, during that incident, if you'd been getting your fuel from a small airport that didn't sell a large volume of fuel, you'd have been pumping clean fuel while the big places that took frequent deliveries would have the contaminated product.