Continuity mistake: At the end, the computer produces a print-out with two lines of type at the bottom, then pauses. In the next shot, when it starts printing again, the two lines have disappeared.
Continuity mistake: When Number 6 falls on the beach, the sand under him is wet. In the next shot, it is obviously dry.
Continuity mistake: Number 6 pushes the dead body out into the surf. In close-ups, the body is still very near the shore. But in long shots, it is much further out. (00:28:30)
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)
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.