Hard Cast Emrakul, the Promised End did not give me control over my opponent's turn
I just hard cast Emrakul, The Promised End but did not receive control over my opponent during their turn.
The upkeep step passed and they were able to cast a spell during their main phase. Unless I missed something, that shouldn't have been possible (I've successfully cast Emrakul before).
Thankfully, my opponent recognized this I think and scooped.