Continuity mistake: When Number 6 is looking for a map in the Village store, the shopkeeper wears a badge with 19 on it. A few shots later, his number has mysteriously changed itself to 56.
Continuity mistake: Number 6 is looking out his window as the maid runs down the stairs. As he watches her, two tables in his apartment change places with each other between takes. A large table with a statue/sculpture on it switches positions with a smaller table on which his lamp and telephone rest.
Continuity mistake: When Number 6 climbs to the top of the bell tower and looks down at the Village, the beach below him changes remarkably between shots. The sequence was obviously shot at different times of day, as the tide is in during much of the scene, but in several intervening shots, the beach sand is completely exposed.
Continuity mistake: When Number 6 observes the stone boat, the tide appears to change from high to low between shots.
Visible crew/equipment: Watch the bottom left corner of the screen when Rover turns around and begins pushing Number 6 away from the helicopter. A film crewmember's head (probably that of the technician operating the weather balloon that played Rover) appears briefly in the shot.
Continuity mistake: When he's trying to escape, Number 6 hides behind a bush with long narrow leaves. But when the camera angle changes to his point of view, the bush now has short broad leaves instead.
Continuity mistake: During the scene with Number 6 and the young woman in the stone boat, Number 6's jacket changes from one shot to the next. The design of the white piping is distinctively different.
Visible crew/equipment: When Number 6 holds up the radio, the camera angle tilts just enough to show us a brief glimpse of the top edge of the studio set.
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)
Other mistake: When No.8 has been caught by "Rover" in the sea, she is pulled back unconscious to land by one large Rover and 2 smaller Rovers (one on each arm). Her legs are seen to kick (swimming style) to help them move. (00:17:25)
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 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)
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)
Continuity mistake: Number 6 chooses a rather thin tree to cut down and fashion into his canoe. Somehow, when he's finished hewing out the wood to form the boat, it's considerably wider than the tree he started with.
Continuity mistake: After Nadia and Number 6 are placed into the packing crate, there's a shipping label on the front. In the next shot, it's vanished. Later, it reappears, but is now on the crate's side.
Continuity mistake: Due to a very poor edit, an extra walking past Number 2 and Number 6 as they talk on the patio disappears in mid-shot.
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.
Factual error: The truck transporting Number 6 and Nadia drives on the left side of the road. Fine for England, where the scene was shot - but they're supposed to be in Poland, where everyone drives on the right.
Continuity mistake: Nadia's number badge is upside down when she serves Number 6 breakfast. It's turned itself right-side up when she leaves, though.
Answer: It's even more obvious than you think, you know who number 1 is in the very first episode. When 2 replies to the question "who is #1?" Change the way he answers from you are number one (in the monotone or accented answer to, "You are, number 6. The comma gives you the answer. #6 is #1. It's the tone of the answer.