I played through the 100 turns of a Mongol scenario and didn't conquer the number of civs I was supposed to (5 for King in my case). But I didn't get the "Khaaan!" achievement either ("Run out of time to beat the Mongol scenario").
Does anyone know if it's bugged, or if there's a trick to unlocking it? I googled around a bit but wasn't able to find an answer. I'm not using any mods and I had an active Internet connection when I finished the game, can't think of anything else that would prevent it.
Right now Steam says 0.9% folks have unlocked it: http://steamcommunity.com/stats/CivV/achievements/