Number 39 gives Number 6 a quiz. When the two of them are being watched by Number 2 through one-way glass, she's writing in a notebook. When it's a close-up of the two of them, she's too focused on looking at Number 6 to be writing anything.
The giant chessboard on the grass is missing in early establishing shots.
During the opening of the chess game the move "Knight to Knight's Bishop three" is announced. There is no such move in chess.
No 6 ticks people off on a chess article. However, it is clearly pasted onto another newspaper, as the paper is a different colour to the surrounding articles, which discuss Kidderminster and road deaths in the UK.
When Rook disables a rotating surveillance camera, he has to lift it up to get at the wires. However, when footage from this camera is shown in the control room, its angle doesn't change at all.
When Rover floats by at the beginning of the episode, there are several shots to show people freeze. One shot is of the plaza, and it's reused footage from Arrival (1967), showing frozen people during a Rover encounter. Number Six can be seen wearing his original outfit.
After the chess game, Number 6 and the rook have a conversation near the chessboard. They look over at the chess game currently in progress, which is actually reused footage from the previous chess game and has Number 6 visibly on the board.
When No 6 tries to knock out the spotlight, he throws one of the guards off the top of the tower. As can be seen in the title sequence and overhead shots, the tower is up on a hill, a bit inland, and quite high. However, we hear a splash shortly after the guard goes over the edge.