Councilman Tarrlok wants to rule Republic City, but the Equalists - an organization of Badass Normal revolutionaries seeking to get rid of bending - are causing major trouble for the place. You'd Expect: Tarrlok to put aside his personal ambitions for the time being and focus on ending the Equalist threat. To that end, he should work on mending his relationship with Korra, and avoid doing things that would antagonize the non-bender population of the city, i.e. the Equalists' main support base. Instead: Tarrlok acts like a Jerkass towards Korra, bitches at her and her friends when they stop an Equalist breakout attempt before he could, and introduces laws that specifically target non-benders. Then, for no apparent reason, he cuts off power to a non-bender part of the city, so that when the residents understandably try to protest against this, he has an excuse to arrest the lot of them. Result: Tarrlok ultimately has his bending taken away by Amon.
The episode where the poor man's gundams were introduced. Having established that metalbending doesn't work and they're too strong to use cables on. You'd Expect: Lin and her police would use earthbending (which is pretty much a prerequisite for metalbending) to at the very least upset the balance of the things and make it easier to pull them over. Instead: They continue to try and overpower the mechas with the cables and get their asses kicked because of it.
Episode 10: Lin is fighting the Equalists using a combination of earth and metal bending. Cue the arrival of the Lieutenant, a guy she's met before and knows for a fact has an electric current running through his kali sticks. You'd Expect: She would use earthbending or grab his ankle with her cables to take him out then and there. Instead: She uses her cables to grab his sticks with metalbending, getting herself electrocuted in the process.
Amon's supposed backstory is that a Firebender killed his parents and permanently scarred his face, so he always wears a mask. You'd Expect: He'd actually burn his face to commit to what he was going to have to deal with his whole life, or explain that a waterbender healed his scars, or not even bothered saying his face was scarred, or literally anything other than what he actually does. Or Better Yet: He could use an abridged version of his actual backstory, e.g. "I had an abusive waterbender for a father." It would demonize benders just as much as his other story, and he wouldn't have to worry about it being disproven. Instead: He applies fake scars with water-soluble makeup. If even the slightest bit of water gets on/near his face, he'd have to out himself as a waterbender to avoid it, and/or be outed as a fraud. All of these things happen.
The Lieutenant walks in on Amon bloodbending Mako and Korra, proving he's been spouting lies to the Equalists. You'd Expect: The Lieutenant to alert the other Equalists of Amon's bloodbending statues, or at the very least perform a sneak attack on him. Instead: He charges head on, and gets pathetically thrown into a wall.
When Wan sees Raava and Vaatu fighting, Vaatu (who is the evil spirit, but Wan doesn't know that) tells Wan that Raava has been bullying him and needs help. You Expect: Raava would immediately contradict him and tell Wan that Vaatu is evil and she's trying to stop him. Instead: She tells Wan not to interfere as it doesn't involve him and Wan frees Vaatu thinking he's helping. note But her opinion of humans was low in the first place, so she likely thought this would cause him to help Vaatu intentionally...
Unalaq's entire plan involves bonding with Korra in order to have her open both spirit portals, something only the Avatar can do. She's already accepted him as a trusted teacher, and has opened the Southern spirit portal on his advice. All he needs is to have her open the Northern Portal before the upcoming Harmonic Convergence, and Unalaq You'd Expect: Unalaq would calmly escort Korra to the North Pole quickly and without incident, where she could then open the Northern Portal. After all, he's on a deadline. Instead: He has the troops of the Northern Tribe invade the South in his effort to make it more "spiritual", asks Korra to remain "neutral", and bides his time. There is nothing presented later on in the story that even suggests that his plans benefit from this action. This act ultimately drives a wedge between the two of them, complicating his plans. You'd Expect: That since Unalaq's plan can only succeed with the Avatar's cooperation, he would make it his top priority to keep Korra on his side. Instead: Unalaq invades her home, has her parents arrested, and her father sentenced to life imprisonment. Unsurprisingly, this turns Korra against him. Result: Unalaq still manages to force Korra to open the Northern Portal, but only because he gets incredibly lucky.
The season 2 finale has Mako, Tenzin, Kya, Bolin and Bumi making a last stand against a huge horde of hostile spirits. Despite holding their own and later being backed up by two Waterbenders (former foes Desna and Eska), they're slowly forced to retreat towards a giant tree with a large hollow space inside. You'd Expect: Bolin and the three waterbenders, the people whose bending abilities are the most limited by their immediate environment, to move away from the tree and attack the spirit horde on its flanks, from terrain they can bend on. Instead: The four of them retreat into the tree, where there's no earth or water for them to use! As a result, Mako and Tenzin are the only ones fighting from that point on, with the other five (Bumi is a Badass Normal) unable to help them.
The Krew, along with Tenzin, Jinora and Bumi, are travelling through the Earth Kingdom looking for the new airbenders. During their travels, they recruit an orphan kid called Kai, and learn that he was previously a thief. While travelling, Kai steals Bumi's wallet, seemingly as a way of playing around, but he initially refuses to give it back. You'd Expect: That after this incident, the Krew would figure out that Kai has not abandoned his old habits, and that someone would take him aside to warn him, in no uncertain terms, that he will be severely punished if he ever steals again. It also wouldn't hurt to keep a very close eye on the kid. Instead: They do absolutely none of the above. Kai subsequently abandons the group, goes on a robbing spree in Ba Sing Se, strands Mako and Bolin in the Lower Ring, and ultimately gets himself captured by the local Secret Police.
Ba Sing Se is in the middle of a riot after the Earth Queen's death. Entire buildings are being put to flame. Mako and Bolin's extensive family wants to leave but dear old granny doesn't want to leave the home she raised her family in, even as it burns to the ground. You'd think: They get the hell outta there, forcing her to come with them (which is what Bolin ends up doing) Instead: They just sit there, waiting to die because some senile old bag doesn't wanna leave.
The Book 3 Finale gives us a grand one courtesy of Zaheer. His plan involves poisoning Korra in order to trigger the Avatar State to protect her life, allowing his team to kill her and end the Avatar Cycle. You'd think: Zaheer would take extra care Such as placing Korra in extra secure restraints, as the Earth Queen's forces had done a few episodes prior. Instead: Zaheer merely has Korra's wrists and ankles chained, which results in her gaining enough leverage to pull the chains out of the walls even without the use of the Avatar State... something Zaheer would have his rear end handed to him by shortly thereafter.