Over the course of Season 5 we see Qyburn save Gregor Cleagane from the poisoning he suffered at the hands of Oberyn. This process turn him into what fans of the show have dubbed him "Zombie Mountain". This is fairly accurate as we see that the the knight formerly known as the Mountain does not speak and has what appears to be skin discoloration.
The first time we see this version of Gergor is when Cersei returns to the Red Keep after her imprisonment. Qyburn introduces the newest member of the Kings Guard as "Ser Robert Strong". This makes sense as Gregor was supposed to be dead and the Martells still wanted his head for the crimes he confessed to. It was originally Tywin's plan to honor this request (but we all know how THAT ended...).
However, we see in Oathbreaker (S6E3) at a small council meeting that Kevan Lannister, Mace Tyrell, and Pycell are referring to Cersei's new guard as Gregor Clegane... mentioning what an abomination it is.
Is Ser Robert Strong = Gregor Clegane the worst kept secret in Kings Landing?