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.
The Avengers (1961)
1 revealing mistake in The Gravediggers - chronological order
Starring: Patrick Macnee
Too Many Christmas Trees - S4-E13
Continuity mistake: When Steed and Emma are driving to the estate, the car's windscreen keeps disappearing. It's there in full shots, gone in close-ups, and reappears again every time we cut back to a long shot. (00:10:30)