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?