Once at an Arco station in the US some years back, I got a tank full of water instead of fuel from the pump.
Arco payed for the mechanic costs to cleans out the fuel system.
What happens is that underground tanks have to be vented and there are ways for water to get in when the station is poorly designed to handle water runoff at grounds level.
Density of Water > Density of Gasoline
Water Goes to Bottom of Tank
Pumps Pull from Bottom of Tank
Bad gas is not always a result of sinister station ownership - it is most likely just poor design. When I lived in Venezuela there was a small station around the corner from where I lived and the guy was going to get shut down for the same problem. He couldn't afford to dig up his tanks so he just shortened the pipe into his tank to keep it pumping above the water level and ran another small line down to the bottom. Once a week he would pump that line until he got gas.
Then he knew he would deliver gas, not water to customers.I liked his ingenuity...