When Pasold rides up to the 6th floor the lift operator is an older man with white hair, but in the shot down the corridor as he exits the operator has become a much younger man with dark hair.
During the fight scene at the end the tape recorder on the table is an actual Philips unit in tight shots, but is just a mock-up box in distant shots as seen by its shape presumably to prevent it from being damaged.
During the last prison scene the guard's voice on tape is speaking in Cantonese ("quickly, quickly") when Red Chinese troops would always be speaking in Mandarin, the national language.
Obvious stunt double for Steed during the fight with the two Chinese guards.
One of the Chinese guards fighting Steed (and Mrs Peel earlier in the episode) is clearly just a Caucasian stunt man.
As Chessman is explaining his organizations's services Pushkin makes a comment regarding the escape of Dr. Wadkin. Chessman attempts to dismiss the slip up by explaining that it was a minor "chink" in their security. The cover for Chessman's operation is that the scientists are being kidnapped and brainwashed by the Chinese.
In the scene where Steed first meets Mr. Chessman, a crew member's arm is visible as he tries to hide under the table out of the shot.