not being properly seated in, or having fallen out of, the cable guide under the bottom bracket,
I thought of that, but the fraying was up near the FD attachment point.
When I was fiddling with it once, I reinstalled it and suddenly the shift lever could keep rotating forwards much furthur, so I took it off the mount again and tried a different orientation for the parts, and this made the lever again stop when parallel with the downtube.
Right. It currently can rotate 270 degrees -- from back parallel to the downtube to forward and 90 degrees from the downtube (pointing torwards the tire). I've never removed the shifters however, so it would have to be something that broke.