Fuel goes bad/gets contaminated mainly by absorbing water from the air. So partially used jugs of fuel that are left to sit for a long time can do that. I've had the last 15-20 ounces of fuel go bad in a jug before when I had drained my tanks back into the jug multiple times. Exhaust by products and who knows what else changed the color and made my engines run erratically. Now I put unused fuel in a separate jug and use it for soaking gummy engines.