Continuity mistake: Monique's position on the floor reverses while she's unconscious. When she first collapses, her feet point to the corner of the room. When she comes to, her head is toward the corner. When she gets up, her feet are oriented toward the corner once again.
Continuity mistake: The towel in Number 6's locker changes colors. When we first see the locker opened, the towel is white. A few shots later, when it's opened again, the folded towel has turned blue.
Continuity mistake: The cape Monique is wearing disappears when she's in the watch shop, then reappears a few shots later.
Continuity mistake: A sleeping Number 6 is being observed by the Village girl. While she watches him, his bed clothes mysteriously rearrange themselves. First they're disarrayed, then they're neatly tucked over him and his robe has moved to a different place on the bed. A few shots later, when he wakes up, his pillow disappears.
Continuity mistake: When Number 6 buys soap and candy from one of the Village vendors, he removes his payment card from his pocket three different times without ever having put it back.
Continuity mistake: During the scene in the bell tower, watch the beach in the background. In one take, the tide is in. In the next, it is suddenly at low tide, with much more of the beach exposed.
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.