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 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.
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.