By feeding Quote Ma Pignon in Curly Story, it will trigger his only line in the game: "Curly! It's me, Quote!"
Contributed by Gamer#1
In the Steam, Wii, and DSi versions, Quote switches outfits based on difficulty settings and during holidays such as Christmas.
Contributed by Gamer#1
When using the Blade at Lv. 3, a ghostly image of King appears, holding his sword while slashing multiple times; it is most likely his brave spirit remained in the sword, fighting to no end to eventually kill the Doctor. This is proven in the description of the weapon in the Nicalis translation.
Contributed by Gamer#1
If the Doctor is defeated and Quote manages to get back to the Plantation's passage, the four statues of the Demon Crown's owners will be replaced with four statues for the Island's heroes; Quote, Curly, King, and Toroko.
Contributed by Gamer#1
It is believed by some that Arthur originally used the sword that King has, entrusting it to King when Spoiler:he was killed by the Doctor much in the way that King gave it to Quote.
Contributed by Gamer#1
After the battle with The Doctor and the Muscle Doctor, if the player speaks to Chako several times, she will mention how Quote is heroic like Arthur, which causes Hearts to appear above the other Mimigas' heads. This is the only way to replenish health before the Undead Core battle.
Contributed by Gamer#1
It's possible to be hurt and even killed by Toroko while she's blindly attacking Quote in the Shack. In Cave Story+, if the player is killed by Toroko in this manner, they'll earn the Toroko Wins! achievement.
Contributed by Gamer#1
On the official Cave Story 3D website and in the Cave Story DSiware guide, Quote is called "Mr. Traveler." This is a placeholder name to prevent his name being spoiled, derived from Chako calling him so, after defeating the Doctor in his human and demon form.
Contributed by Gamer#1
An item called "Chaco's Rouge" ("Chako's Lipstick" in the Aeon Genesis translation) can be found when entering Chako's house. After examining her fireplace and then immediately sleeping in her bed, the item is added to the player's inventory. Chako will also be seen asleep on the bed and will awake if Quote reenters her house. The item has no use.
Contributed by Gamer#1
In one room, called the Statue Chamber, are statues of all of the previous wearers of the Demon Crown, including the current wearer and antagonist of the game, The Doctor. However, if the player completes a series of complex tasks, they will eventually come back to this room after going through the Sacred Grounds and defeating the Heavy Brace boss. The statues are now destructible, and when destroyed, will appear as Quote (the player character), Toroko, King, and Curly Brace, the main characters of the game.
Contributed by broenheim
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In the original Cave Story, Cave Story +, and Cave Story 3D, there is an item called the Beast Fang that is impossible to obtain through normal means. It is obtainable through hacking, or abusing a glitch in Cave Story 3D.
Contributed by crazy7s
Jenka's theme was based on the Finnish folk song 'Letkajenkka'.
Contributed by shaggytoad
In Grasstown, Santa will tell you that the only way forward is through Chacko's house. Chacko will not only help you by telling how to get through, but you'll also be able to sleep in her bed. This will give you her lipstick and a view of her sleeping in bed. The lipstick has no use.
Contributed by shaggytoad
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In the original PC version of Cave Story, Balrog used to make his appearance breaking through a wall and saying "Huzzah!". However, in the WiiWare and 3DS ports, that sentence was changed to "Oh Yeah!" to reference the Kool-Aid Man.
Contributed by Gallego13
During development, nearly all of the enemies were based on bars of soap. The design of these enemies were the basis for Balrog's design.
Contributed by gamemaster1991
The game took the one-man development team, Daisuke Amaya, five years to make.
Contributed by gamemaster1991
Right after the fight with Curly Brace, you can go into her room. In there, you can find a secret tunnel. At the end, you will find an item called "Curly's Underwear." This item is a joke item, as it has no use and cannot be removed from your inventory.

In the Plus versions of the game, finding this hidden item unlocks "Curly Story".
Contributed by gamemaster1991
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There is a portrait image of Sue as a human within the game. This is never used, but it suggests that sometime during development, Sue, in her human form, had dialog.
Contributed by Zackee
In late 2011, a .zip file was uploaded to the Doukutsu Uploader that contained all 19 versions of orgview made throughout Cave Story's development timeline. Dozens of different beta versions of songs that play in-game, including some unused ones in the final version, were part of the package.
Contributed by banshiryuu