[Jack Simth]

As far as I can tell poking around in the ftherlnd files, province population is actually a signed 16-bit integer; but an extra 0 is appended at the end whenever the population is displayed - thus that 300,000 pop province had it's populaiton stored as 30,000 - a 4% increase would mean it goes up by 1200, putting it at 31,200 (stored) - not an immediate problem, if the 300,000 population figure was exact. However, if the population was much over 315,060 (31,506, stored) then that extra 4% pushes it above 327,670 (32,767, stored), which is the absolute limit that a 16-bit signed integer will hold. Beyond that it's overflow. Note that after you reached 300,000 population, it would only take 2-3 turns at 4% growth to have the same problem.