Other mistake: When Aang is reading the letter from The Guru, as Sokka says, "What's a guru? Some kind of poisonous blowfish?" his right eye loses its white colour three times during the shot. (00:18:19)
Other mistake: When the group goes to hug Sokka before he and Aang go to Chameleon Bay, you can briefly see that Toph is incorrectly drawn with arrow tattoos on her hands, like Aang has. This happens very fast, but you can still catch it when watching the episode normally without pausing or slowing it down. (00:20:35)
Chosen answer: Toph had Katara use it to send a letter to her parents. It never returned.