A Man's Teeth Are Not His Own - S2-E13
Revealing mistake: When Rob is "playing" the piano none of the keys on the piano look to actually be pushed down. Also when he's scratching his back with each of his hands the notes on that same side of the piano are still played. He's not actually playing the piano. (Except for the last 2 notes or chords). (00:02:46 - 00:03:27)
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.
Revealing mistake: When Missy steps out the "front door" to call George into the house you can see the backdrop touching the floor. (00:03:50 - 00:05:30)