Visible crew/equipment: When Steed and Lord Melford find the lift out of order, shadows of the boom rigging and microphone make two separate appearances on the wall above their heads.
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)