Why Does Missingno Exist?
For most Game Boy and Game Boy Color games, the fewest possibilities the variables can have is 255 HEX Values, from 00 to FF (the whole process is done in Hexadecimal figures from 00 then 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E and 0F then 10. When 9F is reached, the figure reverts to A0 A1 A2 A3,etc. until the last value, FF). This goes for various data slots in the games such as slots for Pok�mon, attacks, items, letters and standard decimal numbers.
Each Pok�mon is assigned a particular HEX Value, possibly in the order of their creation; for example, Rhydon is HEX 01, Kangaskahn is HEX 02, etc.
However, the slot for Pok�mon only contains data for 151 Pok�mon. This means there are values that are completely empty.*
When an empty value is brought up in a Pokemon battle, the game produces a glitch with the name "MISSINGNO". This stands for Missing Number and indicates that no Pok�mon occupies that particular value.
*Values from C8 upwards contain Trainers (glitched or not). Values from BF to C7 contain glitches otherwise known as the "Rogue Glitches" (see their page). Incidentally, the values from 80 to B9 share the Pok�mon data with letters or text characters, which is why there are other Pok�mon on the Cinnibar Island Strait (see Lvl 100+ Pokemon page).
The Faces of Missingno
Missingno can appear in many different forms. The default form is usually identical to 'M. If you put an "x" "y" or "z" in the 3rd, 5th, or 7th space of your name, though, you can use this to produce Missingno's other forms in the Lvl 100+ Pok�mon glitch. Additionally, you can acces them in the Gambler trick by using a Special of 182, 183 or 184.
How do I find it?
You can find Missingno on the Cinnibar Island Strait (the strip of surfable land on the east coast of Cinnibar island). But first you must go to the Old Man in Viridian and watch him catch a Weedle, or trade with anyone in the game.
Why does Missingno appear?
Two things on this subject can be combined to make sense:
1. When creating the map of which Pok�mon appears where, the programmers made a mistake. The strip of land making up the Cinnibar Island Strait was programmed to be a place where wild Pok�mon COULD appear. BUT... they forgot to specify WHICH Pok�mon could appear there!
Whenever you enter a new area, the data for "What Pok�mon appear where" is overwritten with the specific data for that area. However, because no data is specified for the Cinnibar Island Strait, the previous data will remain, abling you to evidentially catch Pok�mon there from the last area you visited (this is what causes the Safari Zone trick to take place).
2. When you watch the Old Man's demonstration, the picture and text change from your name to "Old Man". For this to happen, the programmers changed the variables given to your name so the picture and text can show up as "Old Man". These are stored in the byte containing the data for "What Pok�mon appear where" as this was the place the programmers would be a safe place to store them *snort* until the demonstation finishes, and they would be overwritten when you entered a new place anyway. BUT... When you go directly from the Old Man's demonstration to the Cinnibar Island Strait, the data won't be overwritten as it should, leaving your name's variables in the "What Pok�mon appear where" byte. None of these variable's HEX Values make sense, so it would fit that the Pok�mon encountered there would not make sense. The game brings up an empty HEX Value = Missingno appears.
What other information on Missingno is there?
There's still the unresolved issue of Hall of Fame data. I don't have the foggiest why this happens, don't ask me. Missingno also shares M's ability to duplicate your sixth item 128 times (see page on 'M for more info). It will also mess up you graphics for a short while, but this usually goes away. If it doesn't just view a Pok�mon's stats screen in battle and the graphics should have corrected themselves.