I just had that happen to me. I was playing as Makedonia and I had a full stack lay siege on a rebel city w/ full stack inside it. On the next turn a 1-unit rebel army shows up and attacks my army. The city garrison is brought as reinforcements but "delayed" (actually happens because there's so many troops that the computer decides to make them delayed) so that it's just the 1-unit army vs my 20 units. Needless to say, that 1 unit turns tail pretty soon. I could have finished the battle then but I decided to continue in hopes that the reinforcements would show up before the routing unit goes past the map edge. Well, they didn't. They just all disappeared and I got an empty town handed to me on a silver platter.

It's kind of hard not to exploit this when it's the AI that attacks you with it's single-unit army