If you declare war and/or close borders with the Aztec Civ on single player you can not attack him because the game thinks you have open borders so your units go right over his units in cities or otherwise. However, this bug may only be confined to this particular Civ because doing it with another Civ does not yield the same problem. Furthermore, the game runs stabily despite this, producing no error messages. Reinstalling does not appear to resolve the issue.