The Northern Air Temple - S1-E17
Plot hole: They specifically say there is a door no-one but an Airbender can open, but then when it is opened, we learn the Mechanist uses it. How did the Mechanist get in?
The Northern Air Temple - S1-E17
Continuity mistake: When Aang, Teo and Katara are walking up to the door of the Air Temple Sanctuary, the pipes which make it open are in a horizontal position, but in the next shot, they are vertical. (00:09:20)
Chosen answer: Toph had Katara use it to send a letter to her parents. It never returned.