Some unused things I haven't recorded before - time links.

0:00 - The map 0B is unused and the game will freeze with a white screen if you go there. Its coordinates are undefined, so when you use the Town Map the game says you're in Pallet Town and the player sprite is placed on the top-left corner of the screen (coordinates: 0,0). I cheated here to change the current map number to 0B. This can freeze the game if you're in the wrong place due to the bank for the current event data changing, but not in Viridian City. The map is only loaded this way when you enter a door with the code on, but the Town Map data changes.
0:26 - When you enter the unused map 0B, the game sets a Fly flag (specifically fly locations, byte 2: bit 4) to say you've been there. In hexadecimal, this value is +08 in address D70C. As the game freezes when you enter the unused map 0B and corrupts the values, this change is only noticeable for a moment. I have shown the change in VBA's memory viewer and recorded entering the map in slow motion so that you can see it without it getting corrupted too quickly. (I disabled the music as it got choppy in slow motion, sorry.)

1:19 - There are some unused options for directions. NORTH/EAST can be seen with GameShark code 01042CD1 and SOUTH/EAST can be seen with GameShark code 01022CD1. These codes only change the visual option, not the function. Their actual functions, if any existed are probably undocumented.

1:39 - The English Red has VRAM tile data on the title screen for "GREEN", which unsurprisingly cannot be seen in regular gameplay. The English Blue does not have any respective data for "RED", though.

1:57 - The Pokémon in Fuschia City's zoo have dialog programmed in. If you use a walk through walls code to talk to them, they simply say "!", like when you talk to the other player in the Cable Club.

2:20 - Dragon is programmed to be super effective against itself, like in Generation II but this is actually unused in Generation I because the only Dragon type move in Red/Blue/Yellow is Dragon Rage, and it deals set damage. I have changed the type of Pound to the Dragon type and used it on a Dratini to demonstrate this.

2:50 - There are some seemingly unused tiles in caves which when put together can resemble shiny rocks, which were probably meant to be icy rocks. They are not part of any of the 4x4 tile 'blocks', but the tile data still exists. I made my own 4x4 tile block using the tool Pokemap to show what the rock might look like when assembled.

3:00 - There is another set of unused tiles in the Underground Path, resembling a lighting fixture. It can be seen in a Glitch City (I haven't tested whether it can be seen in a non-cheating Glitch City yet). In this video, Red bumps into it.

3:13 - Unused items: Coin (3B), false unusable PP UP (32), ????? (07) (a.k.a. 'surfboard'; it allows you to Surf without the move), ????? (2C) (unusable) and TM51-TM55 (FB-FF). I try to use most of them and show their sell price in the Poké Mart. Note that the badge items are unused, because they're used for the Badges list in Cerulean City for the person who tells you about them.

4:17 - Elusive unused Japanese text strings, including unused badge names, Egg/Chick, Bronze/Silver/Gold, Petit Captain, Captain, Petit Master and Master. For more information, see this Bulbapedia article, near the bottom of it. http://bulbapedia.bulbagarden.net/wiki/List_of_items_by_index_number_(GB)

4:40 - Three unused maps are copies of existing maps with the wrong music. I don't know why they exist. For more information, see http://tcrf.net/Pok%C3%A9mon_Red#Unused_Alternative_Maps

5:11 - When you start a New Game the player's name is initiated as "NINTEN" and the rival's name is initiated as "SONY", but when you choose a name these values are overwritten. There is a debugging feature that lets you skip most of the New Game option's introduction too, that can be enabled with the code 010232D7 (010231D7 in Yellow). The default names are different in the Japanese versions. For more information, see http://tcrf.net/Pok%C3%A9mon_Red#Default_Player_Character_Names.

6:25 - There is an unused Poké Mart items list not assigned to any Poké Mart. It sells Great Balls, Hyper Potions, Super Potions, Full Heals and Revives.

6:37 - Prof. Oak (hex: E2) appears as an unused Trainer class. He has a Level 66 Tauros, a Level 67 Exeggutor, a Level 68 Arcanine, and a Level 70 Gyarados. Prof. Oak's fourth Pokémon depends on the roster value (ranging from 01-03). 01 gives Blastoise, 02 gives Venusaur and 03 gives Charizard.

8:50 - There is an unused Trainer class called Chief (hex: E3). He has no rosters defined, so 'grabs' them from the next Trainer class, Scientist (hex: E4). Apparently he has no actual sprite too, so the game uses the scientist's sprite as it's the next valid one.

9:33 - Prof. Oak in the Japanese versions is called オーキドせんせい (Ōkido sensei). This is a different Japanese name for Prof. Oak from when it normally appears. Sensei means "teacher", but Prof. Oak is normally referred to as オーキドはかせ (Ōkido hakase), meaning Dr. Ōkido.

9:54 - The Japanese name for the Chief is シルフのチーフ (Shirufu no Chīfu). This implies that the Chief is related to Silph Co.