Factual error: Major Robinson threatens Steed with a revolver equipped with a silencer. A military officer would certainly know better. Though it's a common movie and TV cliché, it is a glaring error. Revolvers are open-chambered, allowing sound to escape out the back of the barrel - so they can't be silenced. There are rare exceptions if the gun is heavily modified to close the open chamber gap, but that does not apply to the weapon featured here. (00:46:00)
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)