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: Throughout the shooting competition between Number 6 and his Dopplegänger there are numerous shots where both Number 6's are facing the camera and you can see a jittery line running down the middle of the shot where the shots were poorly overlaid.
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)
Free for All - S1-E4
Visible crew/equipment: When Number 6 collapses in the cave drinking den you can see the shadow of the camera cast on the lower left side of the screen as he hits the floor. (00:40:22)
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
Continuity mistake: During the council meeting Number 2's gavel jumps around on his table from shot to shot. (00:16:23)
A. B. and C. - S1-E3
Visible crew/equipment: Just before Number 14 talks to Number 6 (in his dream) you can see the shadow of the camera on the piece of equipment she uses to enter his dream as she walks toward it. (00:25:25)
A. B. and C. - S1-E3
Visible crew/equipment: When "A" confronts Number 6, as he attempts to leave the party, there is a shot facing Number 6 with a doorman either side of him and if you look in the top right of the screen you can see a boom microphone poke out from behind the door. (00:12:48)
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)
Visible crew/equipment: When Number 6 meets Number 2 at the hospital, the shadow of the boom microphone is visible moving around at the top of the screen (on the wall), just before Number 2 greets him. (00:18:53)
Visible crew/equipment: When the unconscious body of Nadia is being dragged onto the beach by Rover you can see the distinct shadows of two crew members and the camera being cast on the ground. (00:18:06)
Visible crew/equipment: When Number 2 and Number 6 are talking at the beach there is a shot of Nadia getting into the sea. Just before she takes her sunglasses off you can see rows of spotlights reflected in them. This is because the scene was shot in a studio and not a real beach. (00:15:25)
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)
Other mistake: In the opening sequence, the zoomed typewriter shows the letter "H" printed as an "X" on the identity card.
Do Not Forsake Me Oh My Darling - S1-E13
Continuity mistake: There are many shots of The Colonel (whose mind has been switched with Number 6's mind) driving KAR 120C all around London. The footage is stock, shown in other episodes, and features either Number 6 or his stand in, but definitely not the Colonel.
Free for All - S1-E4
Continuity mistake: When Number 2 and Number 6 are having breakfast, Number 2's ID badge keeps disappearing and re-appearing underneath his scarf.
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