Do Not Forsake Me Oh My Darling - S1-E13
Continuity mistake: When Seltzman is talking to Number 2, there's a chair beside the tea cart with a foot rest, visible in the shots from behind Number 2. But when the camera angle changes to Seltzman's POV, the same chair appears in the shot - without its foot rest.
Do Not Forsake Me Oh My Darling - S1-E13
Continuity mistake: In Number 2's office, the Colonel stands at the bottom of the ramp. In the next shot, however, he's suddenly beside the tea cart instead.
Do Not Forsake Me Oh My Darling - S1-E13
Continuity mistake: Number 6 pulls up to his London flat and there are six cars parked across the street. When he gets out just seconds later, there are only two cars parked across the street.
Do Not Forsake Me Oh My Darling - S1-E13
Continuity mistake: The configuration of the photos on display in the camera shop changes completely from one angle to the next, depending on whether the camera view is from the outside looking through the window, or from the inside looking out.
Do Not Forsake Me Oh My Darling - S1-E13
Continuity mistake: When the Colonel enters Number 2's office/control room, the doors close as he comes down the ramp. In the next shot, however, the doors are still open.
Do Not Forsake Me Oh My Darling - S1-E13
Continuity mistake: Driving his Lotus through London, Number 6 passes a hearse. But when the shot cuts to a wider angle, the hearse disappears.
Do Not Forsake Me Oh My Darling - S1-E13
Continuity mistake: There are many shots of The Colonel (whose mind has been switched with Number 6's mind) driving KAR 120C all around London. The footage is stock, shown in other episodes, and features either Number 6 or his stand in, but definitely not the Colonel.
Answer: It's even more obvious than you think, you know who number 1 is in the very first episode. When 2 replies to the question "who is #1?" Change the way he answers from you are number one (in the monotone or accented answer to, "You are, number 6. The comma gives you the answer. #6 is #1. It's the tone of the answer.