Continuity mistake: During the opening sequence, a white car parked outside the prisoner's London flat disappears between shots as he drives up in the yellow Lotus and walks to the front door.
Continuity mistake: While the prisoner is packing for his trip (in the opening title sequence), a beach photo placed in his travel bag rotates itself into a different position between shots.
Continuity mistake: In both the opening and closing sequences, as well as in footage used throughout the series, "Rover" leaves the ocean floor as a transparent sphere, but arrives on the surface colored his usual opaque white.
Continuity mistake: In the opening sequence, prior to the Prisoner being spirited away to the Village, the holiday photo inside his briefcase appears to turn 90 degrees between two quick edits.
Other mistake: In the opening sequence, the zoomed typewriter shows the letter "H" printed as an "X" on the identity card.
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