Visible crew/equipment: Watch the bottom left corner of the screen when Rover turns around and begins pushing Number 6 away from the helicopter. A film crewmember's head (probably that of the technician operating the weather balloon that played Rover) appears briefly in the shot.
Continuity mistake: When he's trying to escape, Number 6 hides behind a bush with long narrow leaves. But when the camera angle changes to his point of view, the bush now has short broad leaves instead.
Continuity mistake: During the scene with Number 6 and the young woman in the stone boat, Number 6's jacket changes from one shot to the next. The design of the white piping is distinctively different.
Visible crew/equipment: When Number 6 holds up the radio, the camera angle tilts just enough to show us a brief glimpse of the top edge of the studio set.
Revealing mistake: When Rover attacks Number 6 on the beach, in one of the shots of Number 6 falling to the ground, you can see a tether keeping Rover from roving too far. (00:31:47)
Revealing mistake: Obvious stunt double for Number 6 when he jumps out the way of the buggy car on the beach during his escape attempt. (00:31:01)
Revealing mistake: When the helicopter lands in the field with Number 2 and Number 6 inside, if you look as it sets down you can see a different man (an actual pilot) flying and not the dwarf butler. He isn't even wearing the butler's bowler hat. (00:16:57)
Visible crew/equipment: In Number 2's chamber Number 6 refuses his offer and says "You won't hold me" as he turns around and goes to walk out. If you look at the bottom right of the screen you can see the shadow of the boom microphone cast on the floor. (00:15:20)
Chosen answer: We were never told. In the series finale [Spoiler alert] Number 6 demands an answer to that question, only to be shown his own reflection.
Jean G