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)
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 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: Though the Village isn't supposed to have automobiles (only the little "golf cart" transports), there are several parked cars visible in the shot of Rover floating down to attack the fleeing Villager.
Revealing mistake: At one point, Rover rolls past the Village retirement home, but the film in this sequence was for some reason shown in reverse. Smoke from the building's fireplace is flowing back into the chimney instead of coming out.
Revealing mistake: When Number 6 first enters the bedroom, the light that hangs from the ceiling is moving as though someone has brushed against it. But there's no one else in the room.
Revealing mistake: Number 6 is supposedly flying the helicopter, trying to escape from the Village. There are shadows from the rotor blades crossing his face as he pilots the aircraft. But look closely at the top of the transparent cockpit bubble over his head: you can see the real copter blades - completely stationary and not moving at all.
Revealing mistake: No automobiles are permitted in the Village - ever. Yet in aerial shots, a black car is sitting behind Number 2's residence.
Revealing mistake: When Number 2 and Nadia are in the box, the illusion that they are separated is broken by the edge of Number 2's blanket (and it's shadow) curling round into Nadia's side. (00:39:16)
Revealing mistake: The "metal" door frame in Number 2's office is bubbling and peeling away from the wall.
Revealing mistake: When Nadia and Number 6 are embracing, many of the reverse-angle shots showing them from behind betray the use of a double for Nadia. Nearly every time the camera angle changes, she is noticeably several inches shorter.
A. B. and C. - S1-E3
Revealing mistake: In the fight at the end of the B scenario, Number 6 has a small problem. His pants split open in the back. Fortunately, they somehow manage to mend themselves again.
Free for All - S1-E4
Revealing mistake: When Rover is sent after Number 6, while he is attempting to escape the island in the speedboat, there is an overhead shot of Number 6 in the speedboat and you can see he is being played by a stunt double. (00:29:18)
Free for All - S1-E4
Revealing mistake: Stock footage puts Number 6 in two places at once when he grabs the bull-horn and starts shouting at the Villagers to go. There's an inserted shot of the plaza from "Arrival" that includes Number 6 walking through it on his first day in the Village.
Free for All - S1-E4
Revealing mistake: Though there aren't supposed to be any "normal" automobiles in the Village, there are several visible in this episode, parked in the background. Look closely behind the campaign placards as the golf cart/taxi moves, and behind Number 6 as he leaves the exchange. Several more can be seen behind 6 when the cameraman approaches him.
Free for All - S1-E4
Revealing mistake: Rover flies down from the Green Dome, and briefly reveals his true identity as a weather balloon. His air nipple and guide wires are both showing.
Revealing mistake: When Number 6 lights up a cigar with the other Number 6 you can see by watching the cigar smoke that there is an exact point on the screen where the cigar smoke cannot travel beyond, revealing that there are two shots being overlaid. (00:16:41)
Revealing mistake: When Alison enters Number 2's chamber we see a POV shot of Number 6, Number 2 and the doppelgänger facing the camera. If you look closely you can see jittery lines at various points on the screen were the two Number 6's have been poorly overlaid. (00:26:24)
Revealing mistake: In the aerial shot from the helicopter as it overflies the Village, another "real world" car can be spotted parked next to the Green Dome.
Revealing mistake: Watch Curtis' shadow when he and Number 6 have their pistol range competition. The split-screen process cuts his shadow in half.
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