In general I think more people would be better off in the direction of preparing more for disasters. But here's a case where it didn't work out very well.

In some cases it would be helpful to have extra gasoline. Maybe something happens and you need to leave quickly, but your car is low on gas. Or there's a power outage and you need fuel for a generator. The main downside of keeping a can of gas on hand would be the hassle of rotating it—even with fuel stabilizer gas has a shelf life, so ~once a year you use the gas and go refill. Keeping a can of gas on hand for emergencies seemed worth it: glad to have it if you need it, smallish occasional hassle. I kept five gallons in a new can in deep shade on the north side of our property, outside and away from the house.

I thought of this as low risk enough that when it was time to rotate the gas and our shared car was full, I put it in the tank of a friend's car. They were keeping their car in our driveway after flying out of Logan, and I wanted to refill the tank after driving it a bit. Pretty poor choice in retrospect. When they got back they started driving to VT, got about half a mile and the engine died.

Hours of hassle, towing, and garage later, it turns out that a large fraction of the 4-5gal of gas I put in was actually water. According to the mechanic it was a "shit ton"—when pressed they guessed a couple gallons. I'm still not sure how this happened, but I guess somehow water got into the can and displaced a substantial quantity of gasoline? Past a modern gas can's vapor-tight seal?