I had a game awhile back where Vicky became a runaway AI, and I had to fight my way across half the continent to get close enough to her borders to take her down. So I've got half the continent, she's got the other half. I've been getting closer, and closer, absorbing and capitulating all the civs between us - she can see the storm coming. She's got a large tech lead over me. And when I finally get in position to launch my attack, she's got longbows. She had Assembly Line, and was well up the science line, but never researched Rifling. And her UU is a Rifle unit! Stupid AI.