On certain occasions I am getting lines of rectangular-ish shapes to appear in groups on certain locations of the map.  Hard to explain, see the pics:

Notice they only appear near the edge of the map, along the water:

They are sporadic, because after a turn, the shapes disappeared, only to reappear a few turns later:

Another case of appearance, disappearance, reappearance:

The last shows again how it only seems to appear in blocks along the edges.

I seem to get this when holding the space bar down consistently to pass turns, though I don't believe I was doing that in the first case.

debug.err (for the last case)