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.
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.
Continuity mistake: In long shots of the truck transporting Number 6 and Nadia, the crate they're supposed to be hidden in disappears from the truck bed.
Continuity mistake: Nadia and Number 6 go together to the Green Dome (Number 2's residence). While they stand at the entrance, the door knocker changes to one of a completely different design between one shot and the next.
Continuity mistake: Number 2 sits down and places his umbrella beside him with the handle facing out. The camera angle changes, and although he hasn't touched it again, the umbrella handle is now facing inward.
Continuity mistake: When he's playing chess with The General, Number 6's jacket changes between takes. At first it has solid piping on the lapels, then the piping has gaps, then it doesn't again.
Continuity mistake: When Number 6 and Nadia get into the shipping crate, they're both able to stretch out completely for the 12-hour trip. But when the crate is opened, it's suddenly several inches shorter, and 6 has to bend his neck at an uncomfortably sharp angle to fit into it.
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.