Doctor Who is notorious for promising "game-changing" cliffhangers and in this case it turned out to be true. Missy's reveal in the final moments of Dark Water will go down in history as one of the most memorable cliffhangers in the show's history and that's probably largely because it wasn't preceded by any build ups or gimmicks - it was just a good old fashioned twist which hit viewers in the face when they were least expecting it. It was certainly a catastrophic cliffhanger indeed and it'll be interesting to see if Steven Moffat will ever be able to top it. Apparently he's done it for this year's series, though. He's described the cliffhanger leading into the Series 9 finale as a "whopper" and he's one that he doesn't think anybody will see coming. Knowing Doctor Who fans, they probably will. If it's executed to the same standard as this one, though, it won't even matter if they did.