There's the Demon Slayer Corps in Demon Slayer, and then there's the cooler Demon Slayer Corps, a.k.a. the Hashira. They're not exactly a faction separate from the main private army of Demon Slayers but they might as well be thanks to their manner of clothing and general elite attitude. They're the best line of defense humanity and the Demon Slayers have against the most powerful demons.
With that said, all Hashira in Demon Slayer have one glaring flaw: they're only human. As such, they're not infallible and are certainly not tougher than their demon enemies. This results in some rather lopsided matches and emotionally-charged oversights on their part. As such, even the elite of the Demon Slayer Corps has committed many mistakes both small and severe which have led to some fatal failures or devastating defeats.
Warning: Big spoilers ahead!
7 Voting To Kill Nezuko
It's understandable why the Hashira want to wipe out any demon they see regardless of its animosity or lack thereof. They've lost many comrades to demons. For sure, they're not as compassionate or as considerate as Tanjiro who view some of the demons as victims of circumstance. That's why wanting to kill Nezuko was a failure in terms of their humanity.
Giyu Tomioka is exempted here as he learned not to slay demons on impulse thanks to Tanjiro but everyone else was rather bloodthirsty or nonchalant about executing Muzan's reluctant victim along with her hardworking brother. Had they proceeded with this prejudiced thinking, the series would've been cut short.
6 Losing Kyojuro To Upper Moon Three
The Mugen Train Arc ramps up the show's legacy by finally introducing a Hashira in action in their full glory. Thanks to Kyojuro Rengoku or the Flame Hashira, the fans finally get to see an animated display of Hashira might— and he's no slouch either. Kyojuro is often considered among the top five or around that position in the unwritten Hashira hierarchy.
Sadly, this wasn't enough for him to defeat Akaza or Upper Moon Three in a somewhat fair battle. At best, Kyojuro had to score a Pyrrhic victory by sacrificing himself so that Akaza won't have enough time to lay his fists on the three weaker Demon Slayer juniors. It was an important fight though as it gave a bigger picture of how the war between the Corps and the demons would develop.
5 Sending Kunoichi To Spy On An Upper Moon
Tengen Uzui or the Sound Hashira was a ninja clan leader before becoming a Demon Slayer so that probably explains why he prefers to use his clan's resources instead of the Corps'. Such was the case in the Entertainment District Arc where he sent his three Kunoichi wives to keep tabs on a demon or Upper Moon activity.
This did not end well for his wives. All three of them were captured and could have been killed without producing the results Tengen wanted. Apart from the fact that his wives did not even have Nichirin blades to defend themselves against demons, Tengen could have just gone himself and done the spying (he did so anyway). Not only would that result in fewer civilian deaths but also keep the wives safe.
4 Tengen Uzui Is Forced To Retire After Heavy Injuries
This is where the spoilers begin. So Tengen finally caught up with Upper Moon Six or Daki and the Hashira, along with the three stooges and Nezuko, were nearly able to defeat the demoness until a surprise ally showed up. That would be Gyutaro who's tied with Daki for the Upper Moon Six rank.
This complicated the fight as it was essentially two Upper Moonsagainst one Hashira and some Corps juniors. By the end of the fight, Tengen barely managed to stay alive after defeating the two Upper Moon demons with the help of Inosuke, Zenitsu, Tanjiro, and Nezuko. The Hashira lost an eye and an arm in the process and was also poisoned and stabbed multiple times. Such injuries prompted him to retire which meant one less Hashira for the Corps.
3 Shinobu Kocho Gets Devoured By Upper Moon Two
Shinobu Kocho might not boast much about her accolades and prowess as a Hashira when compared to Kyojuro and Tengen, but she's considered as one of the fastest and most agile among her peers. In fact, when she fought Upper Moon Two, or Doma in the manga, the demon even praised her speed, citing that it was unlike any he's seen prior.
Too bad that didn't prevent Doma from killing Shinobu. The way she died under his hands was also quite cruel; after slashing her across the chest, Doma proceeded to absorb Shinobu. That marked the end of the rather underutilized Hashira as to add insult to injury, the same demon that killed her was also her sister's murderer.
2 Muichiro Tokito Is Killed By Upper Moon One
Very few Demon Slayers have faced Upper Rank One and lived to tell the tale. After all, Upper Moon One or Kokushibo is one of the oldest demons in the series and second only to Muzan in terms of power. He was also a former Demon Slayer who chose to become a demon. That's why it's excusable that even someone like Muichiro Tokito is no match for him.
For the record, Muichiro was the youngest Hashira at only 14 years of age, meaning he was likely going to become even more powerful as he grew older. Kokushibo nipped that potential in the bud when he went all out with his swift and powerful attacks against Muichiro. Muichiro had to sacrifice himself so that Gyomei and Sanemi could behead Kokushibo.
1 Muzan Slays Obanai, Kanroji, And Gyomei
Despite the several Hashira deaths, the score was about even between the demons and the Demon Slayer Corps. Hence, all remaining Demon Slayers trooped into Muzan's castle for the final showdown against him. Despite vastly outnumbering Muzan, the Demon King still outgunned the whole Corps.
Before the final blow was struck, killing the Demon King, Muzan took down with him some heavyweights namely Obanai Iguro, Mitsuri Kanroji, and Gyomei Himejima— the last of whom is crowned as the most powerful Hashira. By the end of the manga, only Sanemi and Giyu were the only Hashira remaining.