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.