Originally Posted by ChrisSlicks

The cheap stations don't deliberately dilute there gas with water (that rumor is as old as gasoline itself) but when the underground storage tanks get old and rusty then water can seep in and mix with the gas. Gasoline is lighter than water so the majority of the water sinks to the bottom of the storage tank, but when supplies run low you are getting stuff from "the bottom of the barrel" so to speak.

Add some stabilizer to the tank top off with some more gas and burn it up and you should be fine.