Revealing mistake: At the end of the episode when Number 6 and Number 2 are addressing the crowd, in the long shots facing 6 and 2 you can tell Number 2 has been replaced with a double. (00:47:51)
Revealing mistake: Obvious stunt double for Patrick McGoohan in some shots during his exercise routine. (00:11:53)
Revealing mistake: After Number 14 convinces Number 2 to let him deal with Number 6 both of them leave 2's office. As they do, you can see the "metal" on the doorframe has started to bubble. (00:30:57)
Revealing mistake: When Number 6 finds the black cat outside his house you can see his shadow fall across the painted backdrop of the town behind him. (00:11:39)
Revealing mistake: When Number 6 is fighting the two smugglers on the deck of the boat you can see creases in what is supposed to be a foggy background.
Revealing mistake: On the raft, when Number 6 starts to take apart his radio, you can see that the raft is in very shallow water but it's supposed to be out at sea. (00:10:25)
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
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)
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: 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)
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