Guys, I thought the FGSS bunch wasn't as ******** as the rest of BF (or the world? or is that the same?)

Here's a few home truths about chain stretch.
Sheldon is right in saying that chains never "stretch" in the strict sense of the word, like an elastic band. The pins and the bushings (or the widened lips in bushingless chains) wear from friction; they develop longitudinal play, if you like. Of course, that does make the chain longer. (Which Sheldon also clearly states. He has nice illustrative photos which I'm too lazy to find now) So lets call it chain stretch and wink knowingly when we say it.

It doesn't surprise me if a brand new chain stretches pretty fast in the first days. During manufacturing, little burrs can be formed on the bushings, etc. These wear down quickly, and the chain settles at its normal length, and starts its slow wear from there. I guess even the generously applied factory grease can keep the pins and bushings slightly apart when you install the chain. As soon as you stomp the pedal, the tension forces it out, and the chain lengthens a bit.

I'm pretty unsure about what happened to the OP. It seems improbable for the chain to have stretched enough to fly off... but then, it was a downhill skip, so there was a lot of force involved. Possibly a small pothole helped in with a big jerk...possibly it was a slightly sideways "hockey skid" that made the frame flex, and effectively shorten the chainstay for a moment. I read somewhere that frame flex does screw with chain tension a lot.

Long, eh?

(Another story chalked up to my "Riding brakeless is not very clever, however zen you are. For example..." list. It's your life, you are entitled to do silly and dangerous things. I do some myself. I'll prolly never do brakeless, though. Being oh so cool is not that much of an objective to me.)