The Decapod - S2-E3
Revealing mistake: In the midst of Steed's conversation with the wrestling referee, the camera loses focus and suddenly whip-pans away when it shouldn't. The actors continue their lines as the camera comes quickly back and refocuses on them. (00:19:45)
Revealing mistake: The secretary turns on the tape recorder to play back her employer's dictation. His voice starts speaking, but the tape reels aren't turning, and the button she pushed hasn't stayed depressed. Apparently, the machine isn't even on. (00:01:20)
Revealing mistake: When Steed comes out from hiding behind the spiral staircase and wall, the rim of his hat touches the wall, which wobbles, revealing a fake wall. It's very noticeable, because there is a shadow cast from the staircase to the wall where you can see it moving.
Man-Eater of Surrey Green - S4-E11
Revealing mistake: When Steed and Mrs Peel are fighting one another, the stunt doubles used bear absolutely no resemblance to the actors.
Quick-Quick Slow Death - S4-E19
Revealing mistake: When Mrs. Peel unmasks the recently-murdered Bernard, the corpse's eyelid twitches. (00:37:00)
Revealing mistake: The dead man in Steed's apartment is visibly breathing. (00:19:00)