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.
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