While controlling my opponent with the cast trigger of Emrakul, the Promised End, I cast my opponent's Cityscape Leveler. With it still on the stack, I then attempted to cast their Mana Drain as well, to counter the Leveler, but was unable to do so. I should have been able to, as there was UU mana available to cast it, but the game would not let me. My opponent also had a God-Pharaoh's Statue on the field, so I'm not sure if perhaps that was bugged and increased the cost of Mana Drain (even though it shouldn't have), but other than that I could not discern why I couldn't cast it.