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.
Do Not Forsake Me Oh My Darling - S1-E13
Audio problem: When Number 6 tells the Kandersfeld waiter that he took some photographs, we hear his words, but his mouth doesn't move.
Free for All - S1-E4
Audio problem: During the campaign, the marching band is striking up a lively tune, and the bass drummer is pounding away on his drum - but there's no drum beat in the sound mix.
Audio problem: Number 48 is still singing when he's carried out near the end - his lips are moving but we don't hear anything. A few shots later, we can hear him again, but his lips aren't moving in sync with the sound.
Audio problem: During a fight with Number 6, one of the Village keepers falls from the bell tower. His fall terminates in the sound effect of a loud splash - but the bell tower isn't anywhere near any water.
Audio problem: During the scene when Number 6 warns Number 2 of a potential assassination attempt on him Number 2 brushes it off and says "it's the little watchmaker that concerns you" but his mouth does not match this. (00:29:43)
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.