I was playing on a huge terra map, as Arabia. I decided to declare on sury, because he was just plain annoying me. So i created a stack of doom with 22 longbowmen i had lying around, 5 catapults, 4 trebuchets, and 7 crossbowmen i moved towards the blue borders and tried to attack with my siege on Angor Thom, but it my cat just moved into the city. then i realized that this was the city lincoln had captured during the khmer/american war, and that both civs had blue borders! so i had left my cities undefended and lost Mecca, Jeddah, and Dammam.
I quickly turned my stack around and took them back while sending other stacks to capture Angor Wat and the other cities whose name i can't pronounce.