The Mountain is no longer played by Conan Stevens (season 1) or Ian Whyte (season 2). He is replaced by Icelandic strongman Hafþór Júlíus Björnsson for the remainder of the series, making him the only character to have been re-cast twice. Whyte may have been unavailable because he also had to play one of the Giants in season 4.
The dialogue between Oberyn and Tyrion was actually Pedro Pascal's first recorded scene. In one take, he accidentally leaned too far backwards against a wooden beam, and nearly knocked a flaming torch out of its holder. Pascal narrowly avoided having the flaming torch land directly onto his shoulder, or lighting his hair on fire.
The episode title refers to the sigil that Petyr 'Littlefinger' Baelish chose for himself, the mockingbird. As he is the only remaining member of his house, it became the sigil of House Baelish.
Since season 7 consists of only seven episodes, and season 8 consists of only six episodes, this episode marks the halfway point for the entire series.
Tyrion cynically mentions that Bronn (Jerome Flynn) and Cersei (Lena Headey) deserve each other. Prior to the series, Flynn and Headey had been in an actual relationship that reportedly ended on such bad terms that each had a clause in their contract, stipulating that they should always be kept apart, both on and off screen. Although Flynn has since denied this in interviews, their respective characters have never shared the screen on the show (save for one very brief moment in Valar Dohaeris (2013)), even though Bronn and Cersei have interacted in the books,.