One Piece has many bizarre characters and locations that some may have difficulty making sense of. For the audience's sake, the series has implied explicit rules in order to help make sense of the otherwise hectic world. It also helps to clarify new powers that Luffy and his friends come by.
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However, there have been instances where the anime's customs are excused or otherwise overtly ignored for the sake of a compelling story. Through identifying such examples, it becomes easier to understand how nothing is set in stone and expectations may be subverted at any given moment.
Logically, age should be an inhibiting factor to one's fighting potential. During the two decades that the samurai waited for the Scabbards to return, many complained they were too old to fight the Beast Pirates.
However, the series' most powerful characters are almost all elderly. Whitebeard, Big Mom, Garp, and Sengoku are all over fifty-five years old. As a result, it's clear that age is not as big of a deterrent to battle as the anime would have one believe.
When mythical zoan Devil Fruits were introduced, they were supposed to be exceedingly rare. However, this phenomenon has become increasingly untrue throughout the events of Wano. Not only does most of Kaido's forces use such a Devil Fruit, but Marco and even X Drake also got their hands on one as well.
Consequently, mythical zoans were ubiquitously featured in one of the series' most important arcs, making the claim that they are uncommon dubious. The only way to resolve such an issue is if no zoan users are featured in the story's future.
When logia fruits were introduced, armament haki was clearly not planned to foil them. The first two users Luffy faced, Enel and Crocodile, both had elemental counters that were necessary in order to win. However, armament haki broke the series' rules through bypassing logia defenses.
Since it is relatively common, logias have become relatively obsolete against anything beyond fodder henchmen. Worse yet, there has not been an instance where logia users were defeated through an elemental counter ever since the events of Skypiea.
When Conqueror's Haki was introduced, it purported to be an incredibly rare form of energy that almost no one could harness. However, it has appeared far too frequently for such a claim to be substantiated.
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Not only does a user have a high chance to pass the gift down to their children (as seen with Roger, Ace, Kaido, and Yamato), it has even fallen into the hands of relatively mediocre characters. Boa Hancock, Doflamingo, and Chinjao are all users of Conqueror's Haki despite being nowhere near as powerful or important as an emperor of the sea.
Throughout the decades, One Piece has followed a consistent pattern where the main villain is defeated by the end of an arc. It has become an unspoken rule, especially given the story's upbeat tendencies.
Whole Cake Island is the only instance where the Straw Hats failed to defeat the main antagonist despite accomplishing their goal. Instead of taking down Big Mom, Luffy defeated Katakuri in a mirror dimension. His victory wasn't particularly satisfying considering that Katakuri only debuted at the beginning of the final act.
Energy steroids were advertised to increase one's strength at the cost of years from their life. Though all new fish-man pirates took and suffered from their effects, it was not in a way that made sense.
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Despite having taken dozens more energy steroids than his minions, Hody was not any worse for wear than they were by the time they got thrown in prison. Should the anime have been more consistent about the pill's consequences, Hody should have died from overdosing on the drug.
When introduced, observation haki was supposed to help telegraph enemy attacks for the user to dodge. However, Katakuri broke the series' rules by using it to flagrantly read the future. He managed to use it so keenly that he could shut down fights before they even broke out.
More inexplicably yet, Luffy somehow managed to imitate Katakuri's exceptional observation haki and develop it himself. This was unlike any other fight in the series and is not consistent with the themes of the anime.
Technically, vice admirals are supposed to be a step below marine admirals themselves. They are treated like legends to such an extent that they are even responsible for committing buster calls and investigating the Worst Generation. Despite their station within the World Government, most vice admirals have proven comically weak.
Momonga was constantly outsmarted and outmaneuvered by Boa Hancock, while Bastille got defeated in a matter of moments during the Dressrosa conflict. Vice Admiral Ronse lost so utterly to Whitebeard that the man did not even need to look at him when taking him out.
There is an unspoken rule that villains either never come back or assist Luffy once they have finally been defeated. This is to make the Straw Hats' efforts feel meaningful without forcing them to kill their enemies. However, Gecko Moria has gone against this trend time and again.
In addition to unleashing havoc on Whitebeard's forces in the battle for Marineford, he has recently been conscripted to join Blackbeard's crew. Given the sinister potential of his Devil Fruit, the evil emperor may devise to return an incredibly powerful servant back from the dead.
In the Enies Lobby arc, it was explained why no single person should have two separate Devil Fruits. The pressure applied to their body and soul would make them explode, killing the host instantly.
Despite this codified rule, Blackbeard has somehow managed to procure the Tremor fruit on top of his preexisting darkness ability. Moreover, he can even steal the abilities of others when giving them to his followers. This was seen with Shiryu of the Rain, whose Clear-Clear fruit once belonged to Absalom. Blackbeard's defiance of nature's laws remains a great mystery.
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