Old Italian steel frame, done up as a single-speed.
Riding a gradual, long uphill, putting good effort on getting up. All of a sudden, at the top and while out of the saddle, I almost go over the bars. The chain had come off and was hanging around the crank. It easily fit back on and I was on my way. No more problems on the rest of the ride.

I have no idea why this happened. The rear, while a quick release, was firmly in the dropouts and hadn't shifted up. Frame flex? The chain is not old enough to be that stretched out.

Ideas?

chain was just to slack probably, check your chainring/freewheel for oldness

How good is your chainline?

The exact same thing happened to me recently. I still don't quite understand why, but I think it could be some frame flex. You were mashing up a hill, possibly the chain was too loose combined with a little flex from the frame, and possibly the wheel moved a little bit in the dropouts. Scary ****. Chainline could have contributed to this as well. Be thankful this didn't cause you a serious injury.

I had a slack as hell fuji touring frame that I rode fixed for a couple years. I had a problem with dropping chains (while riding brakeless) so I got really fastidious with checking chain tension. But sure enough if I hit a bump or rode over streetcar tracks at high speed I'd drop the chain and be on a wild survival ride. As far as I can figure it was frame flex. I got a new frame, swapped all the parts over, and problem solved. Long chainstays with a fixed gear are bad news!

If the chain "easily fit back on," it was probably too loose to begin with.

i'm happy that i got a bmx chain. hasn't stretched out as much.

i've seen the damage from a dropped-chain-gone-bad!

https://www.bikeforums.net/singlespeed-fixed-gear/229296-scary-brakeless-experience.html

If you're "mashing" up a hill, you're probably swinging the bike from side to side. If timed just wrong, the chain will swing to the side of the chainring just as the next tooth is moving into contact. Once one tooth is outside the chain, the rest will follow. This is probably exacerbated by the frame flexing. Try to get the chain a little tighter, and make sure your chainline is ok. The further out of line the chainring is, the more likely it is to happen.

+1

Larger frames will flex more in the rear triangle.

Eureka, what size is your frame?

I see slack chains all the time.
combined with the desire to ride brakeless this is disastrous.

your local shop can teach yhou how to get the tension right.

there should be no "hang" in the chain.

basic tensioners cost a couple of bucks.

the chain ought to be tight enough that you can feel when the chain needs oil because it effects the ride.

I love this forum! After reading all the suggestions and a more detailed review, while chainline looked ok, the chain was slightly slack. Slack enough to take advantage of Sivat's description above. So, a little loose chain and "mashing" (well almost mashing!) and boom.

I moved the wheel back enough to increase the tension and tightened it up real good.

Thanks to all!

BTW: frame is small - 52.