Has that ever happened to anybody? I am riding at a pretty good clip over some rough streets in traffic and all of a sudden pedals spin out. Somewhat of a problem since I have coaster brakes and I'm riding in the rain (don't really want to rely on my back-up front v-brake). Just as I stopped safely I put the chain back on, and all is fine.

So is there something wrong with my (relatively new) bike (KHS Green, btw) or is this something that just happens every now and again, especially if riding over rough roads. Is there something wrong with the chain?

Thanks.

Single speed/IGH or derailleur? If it's a single speed or IGH, probably a tension issue (I've never serviced this kind of bike, so this is only based on a passing familiarity with the mechanism). With a derailleur, it's probably off kilter a bit. Start by adjusting the limiter screws.

I've lost my chain twice. Both times it was an SRAM Power Link that separated, after having been removed and reused. Thing is, I remove and reuse them all the time, I have one link that I took off and put back on again at least a dozen times and never had a problem with it, but sometimes they get cranky and loose for some reason. They always come apart on me when I'm changing gears, I assume the power link happens to drag just wrong across the side of a cog on a downshift.

Right now I'm running my drivetrain to destruction without cleaning it much, so I just permanently joined the chain up with a chain tool.

3sp IGH.

I'd agree with chain tension. If there is room, slide the axle back in the dropouts. If you're all the way back, you may need to remove a link so you have room to adjust.

If the axle nuts are slipping, clean the dropouts and the nuts where they contact each other on both sides of the dropouts and reinstall the wheel, I have even used sandpaper to do this - they must be free of oil, grease, lose paint, etc. If the serrations on the nut are smoothed down, replace the nuts. Also you may want to make sure the locknuts on the hub are locked down.
When the chain comes off, you will need to retension the chain when you reinstall it, if you don't it will happen again.
Also, please learn how to use your front brakes, they are much more effective at stopping than the rear brakes are.

Ok thank you for the detailed advice. Since the bike is only 5 months old, I guess I just bring it in to the LBS.

Btw, I know how to use front rim brakes... so much so that I decided I needed brakes that work more reliably in the rain as well as dry (so much for that ). That's why I bought a bike with coaster brakes. After 35 years of cycling (most of that time on bikes with coaster brakes), this is the first braking failure I encountered with coaster brakes. Needless to say, rim brakes fail frequently, if not as complete as this failure was.

That makes no sense... a front brake will ALWAYS work better than a rear brake!!! And rim brakes don't fail any more frequently than coaster brakes -- either kind will fail if you neglect them.