Just out of interest was the city in question Plavatium ?
I've had a few odd things occur with that city, the most strange was when I had just taken it from Gaul and placed the conquering army in the city and retrained all the units fixed repaired the walls and so and end turn, next turn Gaul place the city under seige, so I decide to sent another army to break the seige and end turn but when its my turn to move Plavatium is greyed out with a gaul flag on it but with city still under seige by the gaul army and when ever I placed my mouse over the city it said Plavatium belongs to Gaul, a bit odd I though but also when I had the fog of war gone wrong prob it was also Plavatium.

LDC