What is the Undefended City Bug?
Occasionally, when I have a lot of cities (around 50), I have one city that shows up as undefended in the map display (no dark border) but in the city display, shows fortified units inside. I usually take all of the units out and then all back in to correct the situation. Sometimes I get the “Romans plunder …” message when I re-enter such a city and the population decreases by one. If a computer civilization attacks a city that is shown to be undefended, they capture it despite the fact that units are present in it. This has been detected in both v4 and v5.