If you already beat the game and found MissingNO., you would notice that if you encounter MissingNO, your Hall of Fame data will be ruined. It'll look like a big mess of pixels, weird names, even Pokemon he never even used before. My friend claims he saw a square-shaped MissingNO. in his HoF, which I believe to be Mew. Why? Here's why:

When I went over his house, I had a look at his Hall of Fame, and that little square had a high-pitched Mewtwo cry, which IS a Mew cry. A few months later I decided to mess up with Gameshark on my Red version. I wanted to catch Mew. Now, people who own GS would know that if you put in a code to find a certain Pokemon, it'll appear as a mess of pixels when you find it, but it'll be normal once you catch it. I used the Mew code on my Gameshark and what did I find? That same block of pixels with that same cry. When I caught it, it was a Mew. Non-glitched, perfectly harmless Mew.

Now, this gives out a lot of info about MissingNO. It seems to me that MissingNO., being a mess of data, interferes with your HoF saved records. That mess of data seems to have some kind of connection with Mew being removed from the game, since my friend saw that same mess of pixels in his HoF without ever using a Gameshark in his life!

So, I believe MissingNO. to be Mew, removed from the game. Want even more proof? Here. The other day I decided to mess around with Pokemon Red again. I caught (GASP) a MissingNO. When I used it, it's back view was a huge block of dark black pixels, and it's only move was Pound. If you know about that Mew glitch in R/B/Y, you trigger things in the game to make Mew appear. The Mew that appears is at Level 7, and it only knows Pound. My MissingNO. learned only Pound! That's why I believe that MissingNO. had to do with Mew being removed from the game, and not just a beta-test.

Why is MissingNO. found?

Whenever the game "sets up" the planned battle against the Weedle, it needs to change the player's character name to "OLD MAN" so that it will display "OLD MAN" instead of the name the player has inputted. The programmers decided to use the area of data where wild Pokémon information is as a temporary storage area for the player's name. When the battle ends, the game will retrieve the player's name from its temporary storage place. The reasoning is that whenever the player goes anywhere else, such as Route 2 or Fly to another city, the game will automatically change that data to what it should be, plus, the player won't actually encounter any wild Pokémon in Viridian.

However, along Cinnabar Island's coast, there is no data for wild Pokémon programmed in, so it won't change the wild Pokémon data to something that makes sense. While surfing the coast, the player's character name will still be the wild Pokémon data in that area. This is probably a mistake on the programmer's fault, for reasons currently unknown.

The name of the player has six hexadecimal values in it. The game needs only three "slots" of wild Pokémon data to store this. The wild Pokémon the player encounters along the coast are determined by the third, fifth, and seventh characters of the player's name, while the levels are determined by the second, fourth, and sixth characters, respectively.

A number of people have claimed that the player must have five or less Pokémon with you if you actually try to catch MissingNO., otherwise, his or her game might malfunction when he or she tries to withdraw it from the PC. This seems to be true only for capturing the 'M glitch. A MissingNO can safely be deposited, but an 'M can't. If the player deposits 'M, the player's game will freeze whenever he or she tries to withdraw one of their Pokemon. This problem can be solved if the player accesses his or her saved game through Pokemon Stadium 2 (which sees any glitch Pokemon as Dittos). The player should simply move the offending glitch from the box into his or her party, and, the player should be able to access his or her stored Pokemon again.

If the player captures an 'M, instead of the battle ending, there will be an invisible Pokémon battling the player. (The invisible Pokémon is really a Ditto.) If the player tries to capture the now-blank region, it will be as if the player has captured a Ditto and were fighting it all along.

In Pokémon Red, when a Missingno appears, it is usually either at a level below 10, or a level above 100.

In Pokémon Red and Blue, if the player captures a Missingno over level 100, and the Pokemon gains any experience in a battle, it will say "MissingNO. has leveled up to level 100". This is the only known situation in the Pokémon games where a Pokémon can go backwards in its levels and experience.

MissingNO. is one of many glitches that are in the Red and Blue versions. Although similar, there seems to be no direct connection between Missingno. and Glitch City, another glitch that can be accessed. MissingNO. has acquired much notoriety due to his presence in many Pokémon myths, especially involving Mew. While the myths aren't true, Mew's popularity ensures that Missingno. will always have a place in Pokémon culture.

Both MissingNO. and 'M have been known to glitch up link cable battles. For example, if each player uses an attack on the opposite player involving a MissingNO. or 'M, what appers on one screen will often not be the same on the other screen. This will cause a glitch. However, this has been known to happen in non-Missingno battles.

If an 'M is taught a move like Fly and then is evolved into a Kangaskhan, the Kangaskhan will be able to use fly as an attack, which would not normally be possible. When the Kangaskhan that evolved from 'M uses fly in Pokémon Stadium, it will look like the Kangaskhan is standing on air.

Thanks to this site for more of the information.