Not a sign of real smartness, more an AI-exploit, but still; a game breaker.
In the Modern Age I was at war with Russia. For the first time ever my Capital was nuked by the Russians and I hadn't even fired one myself (I never use nukes). So I got angry and assembled Modern Armor armies and assembled them in a city where the transports were 1 turn before the invasion (stupid, I know).
The Russians nuked that city and my 15 Modern armor armies and the 100 + modern armors, artillery, Mech Infs etc, needed for the invasion, as well as the 30+ transports were annihilated.
It wasn't smart in the sense of anticipating where the danger would come from, it was just simply the matter of the AI knowing the location of all my units and just firing the just fiished ICBM. But still; it was a game-breaker and I gave up.
For the sake of it; I reloaded 1 turn earlier; set sail immediately and won that game. It doesn't matter; in the real game I had given up, so I count that game for myself as a loss.