Maybe ex-enemy units are taking with them some kind of 'can potentially attack tile x' flag which persists until the unit is destroyed? I can't play Civ 5 anymore due to this bug and have reinstalled Civ4. Pity because the original Cililization I played at about 10 years old, and have played and bought every one since. Been a fanboy in other words. I would even consider buying the DLC just to kill this bug if that's confirmed as a fix, but somehow it seems a bit dodgy just on principle. Not sure what Australian law has to say on patches that break gameplay but I have a support ticket in and will post back.