I found my bike today with a flat rear tire. I took the tube out of the tire and inspected it, and after putting it under water, I found a tiny pinhole where the leak was. It is on the rim side of the tube, about where the tube would contact the bead of the tire.
I've heard of people getting tiny wires separating from the tire and causing such problems, but I'm using a foldable tire, not a wire-bead, so I can't see that this would be the problem. I've ran a cloth around the inside of the tire and the rim multiple times, and haven't snagged anything on either, and I've run my fingers over both to make sure that there wasn't something really small there.
The only thing I found was that the rubber along the very edge of the bead is pretty rough in a couple of places. Has anyone ever heard of, or had experience with, that causing a leak, or is it more likely that I just picked up something from the trail that I brushed away when I was checking for it?