Unused content (and translation quirks) remaining in final game:
*MISSINGNO (error handler): Used for blank species data (appears in the species list 39 times) and adds up to a tentative 190 Pokémon. A user from Smogon "Zog" claims he has spoken to Shigeku Miyamoto and had him reply regarding the 190 Pokémon: "Yes; we decided to save the rest of the designs for later".
*BIRD type: A well known unused type (physical, no types immune or resistant to it, neutral damage to everything according to IIMarck.us). It appears (by chance) on 000 family glitch Pokémon (Red/Blue). Still exists in Yellow and all main-series games up to Crystal.
*????? (error handler): Misnomer is "surfboard". It's assigned to [unused] item #07 (removed name?) which acts like the field move Surf.
*POKéMON PROF: The sprite is referenced on the title screen (Professor Oak) but as a trainer he is fairly well known to have three unused rosters; Tauros, Exeggutor, Arcanine, Gyarados and a final form starter Pokémon. With Ditto Trick; can lower opponent's attack stage (starts at 7: neutral) down to 1 (-6) (Venusaur), 2 (-5) (Charizard), or 3 (-4) (Blastoise).
*CHIEF: Unused trainer class, identifier 227. Assumed pre-cursor to Scientist (Scientist is identifier 228). No defined rosters, uses Scientist picture.
*Default player names: NINTEN (player) and SONY (rival), if starting a New Game but skipping the name selection screen.
*Indirect reference to old Japanese Blue Kadabra for Golem trade: Red/Blue are based off the (once special edition) Japanese Blue's game engine. There is a trade in Cinnibar Lab where a man says that his RAICHU "went and evolved", due to a translation quirk. This is because the trades were changed between JPN Blue and international Blue. Instead of the final Electrode for Raichu trade the JPN Blue used a Kadabra for Golem trade instead.
*Struggle's total PP: Struggle has a default 1 total PP, but it never actually depletes.
*Unused characters: We've had these in our Big List for ages, but thanks to "Tauwasser" for clarifying what they actually are. 'l, 's, 't, 'v, 'r, 'm: unused precomposed glyphs. ぁ, , , , , , , : leftover sutegana.
Not documented much at all:
*Trainer (I.D. 200): Trainers start at identifier 200 and end at 247. Trainer 200 (C8) is invalid and was probably never supposed to be a real trainer but the ordering is a little odd because the first real trainer starts at 201 (C9) (Youngster).
*Location #11 (0B): Is a blank formatted map location (apparently found a long time ago, but rediscovered when working on my AreaDex project). HyperHacker said an unused address can be checked allowing you to fly to it. It's data is completely formatted and defaults to co-ordinates (0,0) (North of Indigo Plateau).
*Directions: The words NORTH/WEST/SOUTH/EAST appear in the Generation I but only "NORTH" and "EAST" are used (in the Safari Zone) with other references to directions not being in block capitals. Bassmasta found this by accident after experiencing some Super Glitch like effects (suggests that he may have recently battled a ZZAZZ trainer). Appears in IIMarckus' text dump. Found again by Sonnikku1011 this year.
*Alternative map locations: As with Location #11, mentioned by HyperHacker a long time ago. Rediscovered in AreaDex project. These are three map locations identical to others but using the wrong music.
*CHIKUCHIKU: Meowth346 mentioned this a long time ago on the Azure Heights forums. He thought it was the Anglicization of the nickname of a Beedrill in a JPN Blue exclusive Butterfree for Beedrill trade. Can be found in text dumps.
*Unused PokéMart data: Veekun has listed items "Great Ball, Hyper Potion, Super Potion, Full Heal and Revive", apparently being located in between Fuschia City and Cinnibar Island PokéMart data.
*Unused opponent AI: According to a disassembly of Pokémon Red by IIMarckus, opponents may use X Accuracy and Dire Hit but this behaviour isn't assigned to any trainer class.
*Reference to English versions in French version: According to Prama: (a French Pokémon glitch website), the dialogue string "TM49… TRIPLATTAQUE!" is used, where CT is primarily used instead.
*Deleted maps: Not including location #11, there are 25 of these, with (17) using the Pokémon League map header, (3) using the Victory Road map header, (1) using the Pokémon Tower map header, (3) using the Rocket HQ map header and (1) using the Rock Tunnel map header. I documented these in my AreaDex project.
*Pokémon Green reference in Red and Blue: If looking at the title screen of Red in a VRAM viewer, you can observe the text "RedGreenVersion" at the bottom, maybe implying that you could easily force the game to display "Pokémon Green Version" instead of "Pokémon Red Version"?
Not implemented within the final game:
*Prototype Pokémon names: Those used in US promotions before the English release of Red/Blue.
*(The) TRAINER wants to fight: A "The BROCK wants to fight!" message is used in page 9 of the instruction booklet and a "The MISTY wants to fight!" screen-shot was apparently shown before the games were localized.
*Blaziken257 said on the talk page of Bulbapedia's "Pokémon Red and Green beta" article that a different dialogue string is used when cutting down a tree in the official strategy guide, perhaps along the lines of "[Pokémon] cut down the bush!" but he couldn't remember if this was exactly what it said.
Not documented much at all:
*Unused soundtrack: Exists in the mini-game soundfont (i.e. Surfing Pikachu minigame/ Here comes Jessie & James)
*Unused battle system: Documented this a long time ago back in 2006. Basically it's a battle system where choosing any option except run returns "Hurry, get away!". Grants a 100% run chance, even in trainer battles.
*Sound-font test?: Using invalid sound-font (01) makes the game pause on the introduction screen. Switching it to a valid sound-font plays an "item-get" sound effect. Still haven't heard from this, I think it may just be arbitrary/random.