The following is a document containing every useful fact and stat I've learned about Missingno. If you have any questions about it feel free to ask me:
The following is mostly info on Missingno in Pokemon B/R/Y Version...
Missingno is a debugging Pokemon accidentally left in the game by Nintendo when they produced the game. It was used in experiments for hexadecimal altering and such to see how everything worked out and to make sure the game was functioning properly. It is an unstable Pokemon being, capable of screwing you game badly, sometimes even destroying the whole GameBoy in the process. If the graphics during battle get messy as a result of catching Missingno, your chances of the game glitching is much greater. But let me say this: ALL GAME PAKS ARE DIFFERENT. And because of this, different things will happen in everyone's game, so there is no telling what may happen if you catch it.
Missingno is responsible, apparently, for the game's save file and all of the big details in the game. I am somewhat confused, though, because Missingno was merely programmed for debugging purposes. But perhaps Missingno IS the controller of the game's main parts; maybe Nintendo simply forgot to remove accessability to it.
'Missingno' is short for 'Missing Number', and is the 'missing link' in the game. It has a Pokedex number of 000, thus stating that there really is a missing number about this thing. But what is even more mysterious is the fact that there is a person in the game that states that there is a 'missing link in the world'. Well, looks like this dude is referring to Missingno, undoubtedly. It seems to me many months ago a gamer on this site caught Missingno, and his player name changed to 'Link' and the game screwed-up. I was amazed later to find that his name, changed to 'Link', could have been connected to the 'missing link', also known as 'Missing Number', also known as 'Missingno'. So, alas, that must be why his name changed to Link!
Missingno has this ability to duplicate the sixth item in your item slot when you encounter it. Why? I figured that maybe the reason for this was because of the unstable reproducing hexadecimal residue data that was left behind in the making of Missingno. Or perhaps Nintendo put it in there like that for a reason? I don't know as of currently...
Remember the old coffee dude in Vermilion City, and how to find Missingno via him? Well, turns out that coffee dude is Missingno himself! If Missingno is responsible for all the Pokemon encounters and Pokemon stats, and if the coffee dude can just encounter a Weedle or Rattata out of nowhere, then he must be able to produce Pokemon anywhere, thus, he is most likely Missingno himself. In other words, he counts as a patch of grass. And maybe this has some connection to him being within the process of finding and capturing ++M++ or Missingno alongside Seafoam Islands.
In the newer versions of Pokemon Red and Blue and Yellow, Missingno has... well... changed... You see, Nintendo has recognized that many players have discovered how to find Missingno and capture it, so it has returned the blow by changing Missingno's appearance to look exactly like the ghosts found in the Pokemon Tower located in Lavender Town, without the Silph Scope. This may have been so that the game does not go unstable after having Missingno be caught. Missingno, in the older Red and Blue games, though, still continues to take a pixeled hex mess of a form, as most people find.
I have estimated that Missingno is the shadow of all existing Pokemon in the game, using certain hex codes to make itself appear as a secure, stable being that will not screw the game. But if you take away those codes and make the Pokemon unstable... then the Pokemon will become able to corrupt the game, and reveal itself truly as Missingno. A very mysterious thing indeed, that Missingno may actually be the primary compositional core of all Pokemon cyber existence.
In Pokemon Gold and Silver... Missingno is almost impossible to catch. Even Master Balls have a hard time catching it, leaving without avail most of the time. Sometimes, Missingno will appear in the form of a '?' with a circle around it, but this only seems valid in Pokemon Ruby, Sapphire, FireRed, and LeafGreen.
On rare occasions, Missingno may cause the massage 'Waiting...!' appear right in the middle of the screen during battle... I have not isolated this cause yet...
I think you can trade it to Gold or Silver Version, I don't know for sure... -Darkboarder_77
As you might know, all Bird Pokemon, or Pokemon that have the type flying, have Flying as their second type, never the first. If the type flying was ever the first, for some reason, it would mess up.
Pidgey is the first Pokemon that is completely flying. Charizard was the first flying, but he HAD to be type 1 fire because, well, he's a fire Pokemon. Since Pidgey was the first Flying Pokemon, Nintendo probably tried to make him type 1 Flying. It probably messed up and they had to start over with a new Pidgey, putting Flying in type 2 and replaced type 1 with Normal. But normal has the weakness of Fighting and the first type is the dominating one. But if you use a Fighting move is not very effective. Meaning type 1 Normal is just preventing it from messing up.
Missingno is therefore probably a Pidgey, type 1 Flying (which is the reason it messed up), and the Normal type is just preventing another Missingno.
Missingno was a deliberate glitch added by the programmers of the game to aid them in testing. It allowed them to progress quickly by giving them access to many items straight away. It would appear that the beta version of the game had land paths to Cinnabar Island which were removed. The Box Trick would appear to be for the same reason. Missingno's picture is either a mass of pixels in a backwards 'L' shape, (similar to a Tetris block) or a ghost from Lavender Tower before receiving SLIPH SCOPE. There is also a Pokemon similar to Missingno with a glitched name consisting of block shapes and the letter 'M'.
"I thought that you would like to know that in many UK versions of pokemon red and blue Missingno appears as a small letter "t", only back to front. He has also been known to appear as the Kabutops skeleton too... the same as the one in the Pewter City Museum (upstairs)."
In the Red and Blue versions, Missingno has no gender. However, when a player attempts to trade Missingno to Gold or Silver, the player with the Gold or Silver game will see Missingno as a male or female, even though Missingno (seen by Gold and Silver as Tyrogue) cannot be traded.
"And the way to trigger the REAL Missingno, not M', (I'm not too sure on this one, haven't done it yet) is too trade with any of the trainers in the game, I.e. the girl at the underground path. Then flying straight back to Cinnabar, getting on a water pokemon and surfing. Apparently, you run into the 'REAL' Missingno. {Me- This is partly correct. After trading a pokemon with any other Trainer in the game, you will run into a pixel Missingno on Cinnabar's right coast.}
For those who don't know how to get Missingno, here it is:
1) Go to the northern part of Viridian City, and watch the Old Man's demonstration on how to catch a Pok�mon.
2) Fly to Cinnabar Island.
3) Surf up and down along the east coast of the island until you find Missingno.
(Depending on your name, Missingno. may not show up. If it doesn't, try the cheat above "How to Catch Missingno.")
Romney '08!