The Girl Who Was Death - S1-E15
Audio problem: For some reason, "Londonderry Air" was dubbed over the original song being performed by the actors/singers. Their lips aren't moving in sync with the words at all.
The Girl Who Was Death - S1-E15
Continuity mistake: When the bulldozer is on fire, the raging flames have died down completely in one shot, but are instantly burning brightly again in the next one.
The Girl Who Was Death - S1-E15
Continuity mistake: When Number 6 is trying to stop the rocket launch, the computer console in the center is emitting smoke in close-ups, but not in long shots.
The Girl Who Was Death - S1-E15
Continuity mistake: In the interior shot of Number 6 exiting the lighthouse, the red doors are open. When the shot cuts to an exterior view of him coming out, the doors are closed and he opens them again.
The Girl Who Was Death - S1-E15
Other mistake: When the girl makes her getaway in the helicopter and Number 6 emerges from the bushes and climbs aboard, look at the trees on the horizon as the helicopter takes off and supposedly lands later in another location. You can see that the helicopter takes off and lands in the same field. Also, the shot of Number 6 hiding behind bushes before jumping on the helicopter, and after climbing off it is the same one. It's from a slightly different angle the second time, but it's the same bush.
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.