The first instalment of the Pokemon series, Pokemon Red and Blue, includes a wealth of left over data and glitches; most easily accessible as well.
Missingno. is essentially left over glitch data, appearing as a Pokemon. Missingno stands for Missing Number.
Missingno can be triggered by talking to the Old Man in Viridian City, and letting him tell you how to catch Pokemon. After this, if one flies to Cinnabar Island and surfs along the right hand side of the island without visiting any other areas, the Missingno Pokemon should appear.
Due to the lack of memory on a Game Boy, when the player's name has to be changed to "OLD MAN" for his tutorial, the player's name is stored in the area of RAM reserved for wild Pokemon, since there are none in Viridian City. Ordinarily, the wild Pokemon data will be refreshed when loading a new area. However, since Cinnabar Island has no wild Pokemon data, the player's name remains in RAM there. Missingno and the other Pokemon that appear here are the result of the player's name being loaded as wild Pokemon data.
A standard Missingno usually appears as either a glitch resembling a reverse L shape, one of the ghosts in the Pokemon Tower, or Aerodactyl and Kabutops skeletons. Missingno does not cause any data loss, however it does mess up the Hall of Fame website from the PC. It also clones the 6th item in your backpack past the maximum of 99- even with items which cannot be carried more than once (Pokeflute, Fossils etc.)
Also, different variations of the Missingno glitch can appear, some which can crash the game and erase save data. Some of these glitches and their appearances are determined by the Mew Glitch- a trick in which the rare Pokemon Mew can be caught early on in the game.
The S.S Anne Truck Edit
The S.S Anne Truck is a mysterious piece of scenery, found by surfing to the land on the right of the boardway going up towards the S.S Anne.
Many rumours have sprung up about its connection to Mew and other rare items; however the truck does nothing but act as scenery. Interestingly, Game Freak included the truck in the Game Boy Advance remakes of these games (Fire Red and Leaf Green). On its pier, if the player walks down a bit, they can find a hidden Lava Cookie, unavailable until later in the game.
In order to prevent S.S Anne from sailing away before you have the ability to use Surf, you must trade over a pokémon with the cut move. This allows you to skip visiting Bill and the S.S Anne and return after you have aquired the soulbadge. Alternatively, all you need to do is lose to a trainer on the boat after you have Cut. If you've come too far for this- you can also exploit a save glitch using Surf to effectively surf through the sailor blocking your path to where the S.S Anne would be.
The Surfboard Edit
The Surfboard (appears as ?????) is possibly a test object for a Water Pokemon's SURF ability. The item is unsellable, and when used near water, the player can surf as if they used Surf with a Pokemon. Enter the Gameshark code 01077CCF to purchase the Surfboard at a shop.