The Tale of the Big Why - S1-E8
Revealing mistake: When Gambit is just about to shoot off Steed's door handle, the 'marble' column behind him wobbles visibly.
The New Avengers (1976)
1 revealing mistake in season 1 - chronological order
Starring: Joanna Lumley, Patrick Macnee, Gareth Hunt
K is for Kill: The Tiger Awakes (1) - S2-E8
Visible crew/equipment: A Russian solder peers into a Salvation Army hall. In the shot that follows, from his point of view, there's a very obvious studio light on a tripod in shot.