Continuity mistake: When the butler shows up at Steed's apartment to kidnap Tara, they fight and her culottes are unfolded at the knee. After he has knocked her out and put the last jigsaw pieces in the puzzle, he goes to pick her up to carry her out, and her culottes have been folded up at the knees with no way for that to have happened.
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)