Continuity mistake: After being hit by the amnesia dart, Steed falls unconscious with his bowler hat still on his head. But when the enemy agent approaches and shoots him again, his hat is off and he's lying in a completely different position. (00:21:00)
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)