I was raised from a broken seed

Pointless trivia about Hollow Knight! Spoiler warning!

Before fighting The Radiance, Hornet shouts a voice line that many noticed sounds a lot like the memetic phrase "Git gud!" At a panel, when asked about this, creators William and Ari confirmed that it was pure coincidence, and that the actual word is "Gektu!", though they admitted they don't know what it means.

Mister Mushroom was created because Team Cherry realised that the unique nook that Cornifer appears in in the Fungal Wastes would be left empty and sad once he moved. They wanted something else to put in that nook, and thus Mister Mushroom was born. Team Cherry have also said that they were "100% serious" about the secret Passing of the Age ending involving him, implying that he is not a joke character and that his journey is far from over.

The working title for the game was Hungry Knight 2, as it was initially developed using assets from Hungry Knight, one of Team Cherry's previous games. The title was changed because the Knight doesn't eat anything anymore, though Mister Mushroom's dialogue when you first meet him suggests it is the same Knight.
Mister Mushroom: ...It's nice that you were able to help your friend. So you're not hungry any more? ...Oh. Excuse me.

William has said that his favourite boss is the Dung Defender, because the fight is so fun. This is one of the reasons (the other being that it could be too easy) that a harder version of the boss, White Defender, was added in the Hidden Dreams update.

Team Cherry have hinted that we might see more of Zote in the future, since he has a habit of turning up in strange places.

According to Team Cherry, Hornet is fully-limbed, she just hides her other limbs under her clothes, though it's not clear how many limbs she has. The needle-and-thread weapon she uses is unique, it was made for her, and there aren't many others in Hallownest with the skill to wield it.

It's theorised that the Weaversong Charm was originally intended (in-universe) for Hornet to use, since if you use the Dream Nail on the corpses nearby, you hear the lines "Princess..." and "For protection...".

In early stages of the game, Cloth was named Burlap, and was referred to using male pronouns. In the final game Cloth is rather androgynous, but Team Cherry have confirmed she is female. Her name was changed because the material burlap doesn't exist in Hallownest.

Team Cherry also confirmed that Jinn, The Radiance and Godseeker are all female.

The original name for the Godmaster update was Gods and Glory, but it had to be changed because there was also a mobile phone game with that name.

Broken Vessel was originally named Infected Knight, then later renamed to Broken Wanderer before getting its final name.

The player was originally going to have the ability to dash backwards, but Team Cherry removed it because, in their words, "it kills the flow of combat more than it adds to it".

Fragile Greed was originally named Unending Greed (Glass), and its design looked like a darker blue version of Unbreakable Greed.

Baldur Shell was originally called Propede Shell, since the original name for the Baldur enemy was Scrundling Propede (Infant).

The White Palace was originally meant to have two bosses, King's Golem and Dusk Knight. They were cut for time, but the King's Golem became the Kingsmould enemy.

The Nailmasters and the Five Great Knights were originally meant to be bosses, but again, this was cut for time - the Nailmasters are fought in brief cutscenes, and the Five Great Knights do not appear in the game, with the exception of Ogrim, a.k.a. Dung Defender (though the boss False Knight and its dream version, Failed Champion, is a maggot using Hegemol's old armour). The Nailmasters eventually did become bosses as part of the Godmaster update. However...

There's a theory that Hegemol might still be around, disguised as one of the other NPCs (the most common candidate being Sly). When Team Cherry were asked if Hegemol is still alive, they gave an ambiguous answer saying they weren't sure if he could survive without his armour, but there's a chance he might still be around. When William was asked in an AMA if Sly was Hegemol, his response was simply "!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!"

When asked what happened to Quirrel after the Blue Lake scene, Team Cherry's responses were simply "I dunno" and "Yeah, we're not sure". Most theorise that he died, since the achievement you get in the Steam version when you sit with him at the Blue Lake is named "Witness" and the description reads "Spend a final moment with Quirrel", though even that doesn't necessarily mean he died. The nail he leaves behind is labelled as "quirrel_death_nail" in the files, but this isn't the evidence people think it is, as the same asset is also used in other locations.

Team Cherry have said that they have no idea how the Knight escaped after it was sealed in The Abyss, but explained that it was "drawn" back into the kingdom, which is why the game begins at Howling Cliffs.

Despite the game being called Hollow Knight, the player character is only ever officially referred to as "the Knight", never "the Hollow Knight". A Steam trading card does bear an image of the playable Knight with the name "Hollow Knight", but this is thought to either be an error or a deliberate deception, since "The Hollow Knight" is actually the name of the first final boss. Team Cherry have said that there is a specific name used for the playable Knight for the purposes of translations, but haven't revealed what this name is. Also, the player character is genderless, and is referred to using it/its pronouns in official material. Within the game, the character is only ever referred to with either descriptions or epithets, like "Ghost" or "Vessel".

The Failed Tramway, according to Team Cherry, was an attempt by the bugs of Hallownest to expand the kingdom into unknown territories below, but sadly it didn't work out for them.

When you first meet Sly, he mentions the name "Esmy" in his Infection-induced daze. William explained that Esmy is someone who still lingers in Sly's dreams, though hasn't elaborated.

Elderbug was supposed to have dialogue telling you to go to Greenpath if you talk to him after obtaining the Vengeful Spirit spell. However, due to an error in the coding, this dialogue is skipped and he instead tells you to head to the Fungal Wastes. Here's what he was supposed to say.
Elderbug: I used to think the kingdom below was all dead cold rock, but I've since been told different. Travellers speak of startling variety in Hallownest's caverns. Even just besides those Crossroads there's meant to be an area full with leafy greenery. I told those same travellers about the wonderful grasses that grow around the town. They didn't seem impressed.

If the first time you speak to Cornifer is in Deepnest, he has a unique dialogue where he introduces himself in a scared, paranoid tone.
Cornifer: Hh-hello. I'm Cornifer. I'm a m-mapper by trade. I've tried to chart the dense nest beneath here b-b-but it's proving too dangerous for a bug like me... Vicious little creatures burst out all o-over the place and the passages are a dark, twisting maze. Even with my good head for direction, I-I-I've had enough... Unless you're well prepared, I'd urge you to leave, though if you're feeling strong enough to descend, would you like to buy m-my meagre map?

While Grubs and Grub Mimics look identical, it's possible to tell them apart by using the Dream Nail on them. Real Grubs will always think "...Home..." while Grub Mimics will think "...Empty...", "...Kill...", or "...Other..."