Other mistake: When Azula attacks The Western Air Temple, there is a close up shot of her when she says that she is about to become an only child. Pay attention to the smoke coming from the airship behind her, especially above her head: it doesn't move. Even if this was remotely possible, it does move when the camera zooms out, making it a mistake nonetheless.
Audio problem: At the end of this episode, Aang tells Katara that forgiveness is the first step you need to begin healing. But if you watch closely, his mouth isn't moving at all when he says this.
Chosen answer: Toph had Katara use it to send a letter to her parents. It never returned.