Also worth thinking about - the tank from which you pump the fuel in the first place. One would like to hope that the various airfield operators keep their storage tanks in good condition(?!), but for those aircraft authorised to use mogas, then you probably need to be even more careful.
I had a problem about 5 years ago when I filled my car up from a filling station next door to my office. The car ran like a pig, spluttering, misfiring, etc. At first I thought I had inadvertently put diesel in the tank, but talking to colleagues, they were experiencing the same thing, having used the same petrol station.
My car was due in for a service anyway, and when I picked it up, the workshop had kept the old fuel filter to show me. It was full of bits of scale, and also some other junk. When I told them where I had filled up, they weren't surprised. The filling station was not one of the 'big name' companies, but was a small independent operator who was free to buy fuel from any supplier.
The service workshop reckoned the petrol station was buying contaminated fuel on the cheap because no one else would have it, or their tanks and tankers were rotting away inside, thus contaminating the fuel. Needless to say I didn't use the petrol station again, and didn't have any further problems like that.
Of course, everyone checks their fuel before they go flying. Don't they?