Much of the drama in the anime and manga of One Piece comes as a result of everyone's different ambitions and desires, but some of it comes from the characters making bad decisions, being clueless about their own idiocy and then being punished for it.
A bunch of people ate mystical fruit that gave them superpowers but made them unable to swim. You'd Expect: Them to immediately take a sturdy merchant ship to the nearest continent and live out their lives in a low-moisture area. Instead: They keep living on a string of islands, and half of them work as sailors. Admittedly, this does have some historical justification: even in the real world, pirates and sailors of the old days could not often swim.
Vice Admiral Garp wants to make sure that his grandson, Monkey D. Luffy and Portgas D. Ace, grow up to become a respectable marine, or at the very least anything other than a pirate. Unfortunately, he's unable to raise the kid himself (presumably due to his duties as a marine). You'd Expect: Garp to try and put Luffy in the care of people who he knows will raise the kid right. As a Vice-Admiral, and one of the most important figures in the marines, Garp would presumably have a number of subordinates on hand who could fit this role in some way. He could also make sure he's kept away from negative influences, and work on building up a bond with him, so that he'll have more chance of influencing Luffy's decisions. Admittedly: Luffy is descended from an infamous criminal, but barely anyone in the world would be able to figure that out just by looking at him, and Garp could easily have Luffy be given a fake ID, or something similar. Instead: Garp just leaves Luffy in a backwater village in East Blue. Luffy subsequently befriends a crew of pirates (Garp apparently making no attempt to stop this) and decides he wants to become a pirate. At this point, Garp realises that he made a mistake, takes Luffy out of the village... and sends him to live with a load of bandits, which naturally does absolutely nothing to curb Luffy's ambitions, or teach him respect for the law. And when Garp is around, his interactions with Luffy consist solely of him giving the kid a Hilariously Abusive Childhood, leading to Luffy fearing him more than anything. Result: Luffy becomes one of the most notorious pirates in the world, and Garp has to deal with the very real possibility that he'll either have to fight his grandson, or watch him be killed by the World Government's forces.
During the events on Conomi Island, Luffy is rescued from the pool and returns to fight Arlong. Arlong is aware of the Devil's Fruit users' weakness of being powerless in water, and he had put Luffy into the pool to begin with. You'd Expect: Arlong to throw Luffy into the water again, thus putting him in trouble. Instead: He chooses to gloat over how Fishmen are a higher race than humans, by talking about how humans are helpless underwater, among other things, and then fights Luffy on land.
Upon entering the Grand Line, the Straw Hats' ship is in danger of crashing into a giant whale. They slow down by firing the ship's front mounted cannon, but still sail into the whale. The only damage is the ship's masthead being broken off and the whale doesn't notice them and starts submerging. You'd Expect: The Straw Hats to wait for the whale to go, and continue on with their journey, reattaching the masthead later on. Instead:Luffy, angry at the whale for breaking his favourite seat, punches it in the eye. The whale notices, and swallows the ship in response. Subverted: Believe it or not, this stupidity leads to Luffy learning about the whale's past, and befriending it. It's name is Laboon, and it is waiting on its old crew to return. This becomes very important when the Straw Hats encounter Brook for the first time.
During the Alabasta arc, after having escaped Crocodile's cage trap and making their way across the country, the Straw Hats know, or can guess the top remaining Baroque Works agents are waiting at the capital with orders to kill princess Vivi on sight. Since the rebellion sparked by Crocodile is already well underway, with rebels swarming in the millions towards the same city, they also know they are seriously pressed for time. The Straw Hats manage to draw the agents away with a clever scheme to give Vivi an opening to talk to the rebel leader and explaining the situation, thereby halting/stopping the whole war. You'd Expect: All the princess had to do was run up to them, showing her face to the leader, who happened to be a good childhood friend who risked his life to save her from bandits at one point, and who would be certain to at the very least hear her out, which would've halted the rebellion and maybe stopped it before the fighting got serious. Instead: She stops a few hundred meters in front of them, which at first seems smart, (not appearing as a charging enemy and all) but a mole in the army fires a canonball that lands right in front of her, obscuring the view of both rebels and princess, thus stopping the attempt at stopping the war. It should be noted that Crocodile told her he had moles in the army, and she still stands within firing range of the city, with people who knows what she attempts to do, and has orders to hinder/kill her to the best of their abilities. Then: She gets badly trampled by the advancing army, only saved because her duck throws himself on top of her, saving her from the worst. And the war goes on as planned, resulting in the deaths of hundreds, possibly thousands, of combatants on both sides of the conflict before it is finally stopped.
While in Water 7, Usopp gets jumped by members of the Franky Family, beaten up, and has a large amount of the Straw Hats' money stolen from him, which they were planning to use to buy a new ship. He's understandably very upset about this. You'd Expect: Him to stay on the Going Merry and recover from his injuries. Or, if he really wants to make up for losing the money, ask the other Straw Hats to come with him to the Franky Family's hideout as backup, and take the money back by force. Instead: He decides to go to the hideout on his own. Virtually all of the Franky Family are there, and they beat the crap out of him. AGAIN. Granted, he was ashamed he lost the money in the first place, but that was still a pretty stupid move.
In order to rescue Robin, the Straw Hats need to defeat each of the CP9 and get their keys. Unfortunately for Sanji, the first opponent he faces is Kalifa, a woman. You'd Expect: For Sanji to either drop his feelings for women for once and beat her, or run away and try and find another opponent to fight. Instead: He constantly lets her kick his ass several times. Nami even noted how stupid that was of him. Then: She takes his place and is reduced to hiding with a Mirage Tempo. They both would've been screwed if Monster Chopper hadn't arrived.
Boa Hancock, ruler of Amazon Lily, is one of Seven Warlords of the Sea, as part of a treaty to protect her people and country from the World Government, who have some extremely powerful forces at their disposal. She's called upon to fight for them in the upcoming war with Whitebeard. Admittedly, she has a very good reason to hate the World Government. You'd Expect: Her to grit her teeth and agree to fight Whitebeard, if only to keep the World Government off her back and protect her people. Instead: She refuses, and not only that, she tries to rob the ship sent to get her and attacks the crew. Needless provocation at its finest.
What Chief Warden Magellan did during the Impel Down Arc could probably be considered the epitome of this trope. During said Arc, Luffy, Emporio Ivankov, and Inazuma manage to get down to Level 6 of Impel Down and free Jinbe and Crocodile. Shiliew (a former Chief Guard of the prison who is on death row in Level 6) witness this and tells the officers to pass word to Magellan about what happened, saying that he'll lend a hand with handling the escapees. There's also the problem of Buggy and Mr. 3 causing a prison riot and as if the situation couldn't get any worse, the Blackbeard Pirates have entered the prison and began wreaking havoc. You'd Expect: Magellan would completely ignore Shiliew and retreat, along with all his forces, to the prison's entrance and make a stand there, since all of his enemies will eventually have to go there. In the case of Shiliew, it should be noted that he was imprisoned because he killed prisoners as a pastime, referring to his victims as"just trash". Instead:He. Lets.Him. Out. The Chief Warden of a maximum-security prison authorizes the release of a ridiculously powerful Ax-CrazyMaster Swordsman from Level 6, where the most powerful and dangerous prisoners are kept, because he thought he needed this psychopath's help. Result: Shiliew refuses to play along, joins Blackbeard, and helps him, along with several very dangerous criminals, escape the prison. But worst of all, Shiliew takes it upon himself to kill as many people on the way out of the prison as he can for the sake of bloodlust, starting with the guards who released him. Bonus Idiocy: Magellan had previously beaten Luffy with ease, and goes on to thrash virtually anyone who tries to fight him, thus showing that he didn't need Shiliew's help here.
Magellan's first fight with Luffy. Before the fight starts, Luffy is told that Magellan can poison anyone who touches him. Also, the reason Luffy's in Impel Down in the first place is to rescue his brother Ace, who will be taken away and executed in a few days. You'd Expect: Luffy to either try and get past him, or attack Magellan in a way that doesn't involve him making contact with his bare skin. Instead:He REPEATEDLY PUNCHES the guy. After all, it's what he does best and has worked against everyone else right? So against the guy who can poison anybody who touches him, it'll work without a problem. Result: He's predictably beaten easily and fatally poisoned by the Chief Warden. He only survives thanks to one of his allies having the sense NOT to fight, and said ally being a Determinator in getting him to someone who can help. To make matters worse for Luffy, by the time he's recovered, it's too late for him to save Ace from being taken away from the prison.
The entire battle of Marineford. Ace has been captured by the World Government and is to be executed. This naturally incurs the wrath of Whitebeard and his entire pirate armada, who head to the Marines' headquarters to rescue Ace. You'd Expect: The Marines would simply return Ace to Whitebeard and avoid a devastating conflict the know they will probably lose. Instead: They prepare for battle, bringing as many powerful fighters as they can from around the world to fight the Whitebeard Pirates, highlighting how dangerous their opponent is. Result: Whitebeard, World's Strongest Man, arrives and utterly annihilates everything in his path, nearly killing members of the admiralty. The entire headquarters is wrecked and who knows how many Marines are killed in the battle. Then Blackbeard takes Whitebeard's Devil Fruit power, abandons the WG, and become a new Pirate Emperor, becoming an even more dangerous adversary than the now dead Whitebeard. Whitebeard's last words and the power vacuum created by his death cause the beginning of a new age of piracy.
Luffy and Ace, along with the remainders of the Whitebeard Pirates, are trying to flee Marineford. They run into Akainu, who begins to trash-talk Whitebeard, something that Ace doesn't take kindly to. You'd Expect: They ignore the trashtalk and continue to hightail it out of there to their escape ship. Instead: Ignoring his fellow shipmates' and even Luffy's urgings to ignore Akainu and his words, Ace takes the bait and starts a fight with the Admiral, someone who can injure him despite his usual intangibility, and ends up with a lava fist through the gut in order to protect Luffy when Akainu decides to go after him too. And unlike everyone else in One Piece, he actually dies.
From the Fishman Island Arc, VanderDecken has used his Devil Fruit Powers to turn a gigantic ship into a homing missile, and send it flying towards Princess Shirahoshi. Not wanting the ship to crush her homeland, Shirahoshi flees into the ocean, and the ship follows. Hody Jones is none too pleased with Decken's actions, and starts climbing up a large chain attached to the ship in order to sort him out. Luffy sees him, and wants to stop him from harming Shirahoshi. Remember that the ship is moving further into the ocean, and that not only do fishmen like Hody have the Home Field Advantage over humans while in water, but Luffy has eaten a fruit that renders him unable to even MOVE in water. You'd Expect: Luffy to ask his ally Jinbe to go after Hody and Decken. Since Jinbe's also a fishman, fighting an underwater battle would be no trouble for him, and he's just as tough as Luffy. Instead: Luffy decides to go after the pair of them himself. Sanji manages to launch him onto one of the chains, above Hody. Now this, on its own, isn't too idiotic, but... You'd Then Expect: Luffy to jump onto the chain Hody's on, and knock him off it, and get Zoro or Sanji to take him out and stop him from getting back onto the ship. It's been established earlier that Luffy has Hody completely outmatched in melee combat, and Zoro and Sanji aren't that much weaker than him. Instead: He goes straight for Decken. Although he's able to solve the water problem somewhat by blowing a bubble, Hody soon ends up nearly owning him when they're both underwater, and Luffy only survives thanks to Fukaboshi.
Several of the Straw Hats have switched bodies thanks to the powers of Trafalgar Law. As Franky is heading out in Chopper's body, Chopper instructs him in the use of Rumble Balls: taking one will transform him into a massive monster that will give him immense strength for three minutes, but afterwards leave his body paralyzed for several hours. Furthermore, it took Chopper a long time to learn to control this body at all, and it would have been conceivable that Franky could not have done that either. He is naturally told to eat the drug only under an extreme emergency. You'd expect: Franky to pocket the Rumble Ball and save it for when it is needed the most. Instead: He eats it immediately and goes on a drug-infused rampage.
Usopp, Robin and the Tontatta tribe are planning to take out Sugar, one of Donquixote Doflamingo's executives, as part of their strategy for taking the Warlord down. Sugar has eaten a Devil's Fruit that allows her to turn anyone she touches into a living toy that can only do what she says. The plan is to force-feed her a grape that will render her unconscious. She's normally untouchable due to her bodyguard Trebol being constantly at her side, but some distractions cause Trebol to leave Sugar alone for a moment. You'd Expect: Usopp to take this chance and try to shoot the grape into her mouth undetected, or at least have the dwarves slip the grape into her bowl of identical grapes. Instead: Usopp refuses to fight, claiming that if he does so, he would quickly beat Sugar. And the Tontatta tribe, who severely underestimate Sugar's badassery, consider this a good reason for him not to fight. The Tontatta tribe then pretty much march into Sugar's room, announce their intentions, and Zerg Rush her. Because a little girl who can enslave anyone she touches could never be a dangerous opponent. Result: The Tontatta are all beaten and turned into toys, along with Robin. The plan subsequently only succeeds because the good guys get lucky.
Early on in Film Z, the title character gets into a fight with Admiral Kizaru, where it's revealed that Z posesses a number of Dyna Stones, rocks that create massive explosions when exposed to oxygen. You'd Expect: Z to be extremely careful with these weapons of mass destruction, and not use them recklessly. Instead: He decides to set one off to celebrate his reunion with Kizaru. Long story short, he ends up getting washed out to sea, and could have died if the Straw Hats hadn't run into him.