Do Not Forsake Me Oh My Darling - S1-E13
Revealing mistake: When No. 6 is putting the 4 slides together to get the location information for Seltzman, as the slides are dropped into the top of the projector, they drop down the screen. However, a projector inverts slides, so they should slide up from the bottom. Also, when the last slide is dropped in, the final letters fade into view and not slide down as would be expected. (00:33:30)
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.