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?

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I think this is maybe a good illustration of the problems you encounter when you go from a highly optimized infrastructure (gas stations) to a "simpler" process (gas cans). If infrastructure breaks down - due to war or disaster etc. - you go down multiple levels at once. Even if you think you understand the process there are likely many relevant details that you may not be aware of that can bite you at any point. 

Bullshit. I don't believe it. Gas do not turn into water. I am sorry, but somebody "borrowing" your gas and returning water is more likely explanation. (No special knowledge here, tell me I am wrong!)

Seems pretty unlikely: while the can wasn't locked up, if someone was going to steal it I'd expect they'd just take the whole can and go.

When I was a kid, my little brother stole gasoline (from a neighbor's lawn mower) -- and replaced the stolen liquid with water.

Also, most parents keep a close enough eye on young kids to make it hard for the kid to retain possession / control of something as large as a five-gallon gas can without risk of the parent's finding out that the kid is in possession of something dangerous -- and a kid will probably realize that before stealing a five-gallon gas can.

You are wrong! Ethanol is mixed into all modern gas, and is hygroscopic -- it absorbs water from the air. This is one of the things fuel stabilizer is supposed to prevent.

Given that Jeff did use fuel stabilizer, and the amount of water was much more that I would expect, it feels to me like water must have leaked into the gas can somehow from the outside instead? But I don't know.

I agree with Jeff that if someone wanted to steal the gas they would just steal the can. There's no conceivable reason to replace some of the gas with water.

Read a bit about interaction between gas ethanol and water, fascinating!

Do you mean that the container with the contaminated fuel was stored outdoors in the container that you linked to?

If yes, then a couple things come to mind:

  1. Did you confirm that the container was watertight?
  2. Is it possible that the relatively small size of water molecules meant that they could sneak past mechanisms designed to hold back large hydrocarbons?

Do you mean that the container with the contaminated fuel was stored outdoors in the container that you linked to?

Yes. I put 5gal gas (plus the recommended small amount of STA-BIL) into the linked container, and stored it outdoors, yes.

1: I didn't. It's possible I didn't close it correctly? This seems like the most likely possibility.

2: Modern gas cans are vapor-tight so it seems to me they should be keeping water out.

You may want to try Trufuel. It's available at HD in 2.1 gallon cans. Not cheap ($44 for 2 gallons) but supposed to last 5+ years if sealed.