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The episode "The Earth King" has three whams in a row: The Dai Li remain loyal to Long Feng despite the latter's imprisonment, Toph is lured into a trap by Xin Fu and kidnapped, and the three Kyoshi Warriors are Azula, Mai and Ty Lee in disguise. Granted, these are relatively low-key, but they merely serve as set-up for...
"The Day of Black Sun" parts 1 and 2 - What was built up as the definitive strike against the Fire Nation turned into a colossal failure when it's revealed that the Fire Lord knew about this day long in advance and Azula is waiting for them. Notable also for Zuko's Face-Heel Turn to join the Avatar.
"The Boiling Rock Part 2", when Azula's "friends" Ty Lee and Mai betray Azula at the Boiling Rock. Azula, who has used fear to control them without any problems, is suddenly backstabbed by them.
And of course the way Aang's final fight with Ozai goes. Earlier on, Aang is told by his friends that killing Ozai is the only way to ensure the world's safety. However, Aang finds this to be against the teachings of the Air Nomads - he wants to avoid violence unless it is in self-defense, and even then tries to stay non-lethal. He speaks to the previous Avatars, and they tell him that the Avatar must do whatever it takes to restore balance in the world. Even Roku, who spared Fire Lord Sozin, regrets his choice, and doesn't want Aang to make the same mistake. Aang regretfully accepts what he must do. He later speaks with the Lion Turtle, but we don't know all they spoke about. Near the end of the fight, Aang has the possibility to redirect Ozai's lightning back at him, killing him, but decides not to and redirects it to the sky. The fight continues, and Aang finally knocks down Ozin. It turns out that the Lion Turtle proposed another way to solve the problem. Aang rediscovers an ancient, lost form of bending - energybending - and with his strong, "unbendable spirit", bends Ozai's life energy, taking away his ability to firebend. Ozai without his powers is hardly a match for anyone, and he promptly gets thrown into prison, ending the conflict non-lethally. Well, except for all the Fire Nation soldiers killed in the preceding battles, or in Sokka's plan (dying in war balloon explosions or crashes). But hey, What Measure Is a Mook?