Watching the '#' town Ponderedplunge to the east, move east in Fast Travel. Fairly shortly, the '#' should suddenly drop two population levels to '+'; reversing direction (or continuing on) may result in it switching back to '#', or else changing to '*'.

(I've also observed the sun sometimes suddenly turning into a moon and back again (without changing position in the sky, likewise day changing to night and then back to day upon retracing the last step taken), but don't have a reproducible save of that for the moment.)

For the population matter, this may have something to do with being unable to find any shops now (after killing a shopkeeper?). The '*' town Foughtstrap had become a + town without shops, but I thought that going to the different-civilisation # town Ponderedplunge would allow me to use shops again; however, it likewise suddenly became a + town as soon as I approached it, which seems abnormal. This save is taken right before it switches to a + display (and switching back temporarily is particularly strange, maybe something to do with the transition between the zoomed-in/-out modes of fast travel).

I tried to get a save one fast travel step before the change, but upon saving and loading again I found the # town already displaying as +, thus this save which starts a few more fast travel steps to the west.