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're led to believe Rover is some kind of robot in the series. According to books on the show, Patrick McGoohan was looking for an unusual guardian for the Village, looked up at the sky, and saw a weather balloon, which was what Rover was.
Captain Defenestrator