Only had one issue with supermarket fuel - few years ago there was a problem where a containership wasn't correctly washed down after carrying diesel and that contaminated a load of petrol.
Filled my car at Tesco and within miles the car was spluttering, low power, etc - still didn't initially pout it down to the fuel until it hit the press.
Tesco were very good about it in the end - I took the car in, it obviously had to be emptied of fuel, flushed, filters replaced etc. I had a loan car for the couple of days they kept it - all fully refunded by Tesco, including the cost of the full tank of fuel I'd put in, within 14 days.
Obviously shouldn't have happened, but again - Tesco wont have been the company actually running that process of shipping fuel, storing fuel etc.