Ok, so pretty much what we do is give any info on what we know about missing no. I guess I will start.
I have had a personal experience with it. It is  terrible. Its the rarest and hardest to catch Pokémon in
the Pokémon world. Its a water type, and if you catch it then you are in trouble. If anyone else has had 
any experience with this, please post.

Actually MISSINGNO. is quite harmless to the save file and the game itself. The only harm it can do is something with changing boxes but that is only in Pokemon Yellow when it's "M" and not "MISSINGNO." MISSINGNO. can mess up the battle sprites, but this can be fixed by looking at any other pokemon's status when not in battle.



More facts: MISSINGNO's cry is strange because there is no cry for him. The way the Gameboy is programmed to cope with this is that it will try and search for the proper sound file in the game; to do this, it will do either one of two things:

If no sound file can be found, it will play the next sound file.

Play all the sound files one by one until the sound is found. (Which generally never happens, and thus it will go into an infinite loop, or simply stop)

Huh. You know, I didn't know that, so I thought that the rumors about it totally messing up the game were true. Oh, so does that mean that you caught MISSINGNO before?

epicpokenerd! : Caught? I did an entire Grade seven school project on it! XD.

It was explaining how different pokemon in the game appeared in the famous Cinnibar Island spot after doing the Old man glitch, based on the characters in the players name. Why the game did this, and how it worked. I had to restart my files on Pokemon Blue several times, thankfully Pokemon Stadium's X3 Speed feature is extremely helpful. Either way, things like MISSINGNO. always make me feel really giddy; I love programming mishaps and glitches like him A LOT, and I experiment with such things very, very thoroughly.

Well that's cool. You know, I did a 3rd grade project on it. My sis caught it once, and the next day she ran into a wild lv. 100 dragonite. Is it possible for MISSINGNO to do something like that?

You can get pretty much any Pokemon from Missingno, even ones that you cannot catch normally in Pokemon Blue and would normally need Pokemon Red to catch for, or trade with. Heck, I had a Snorlax that knew Fly AND Sky Attack...


That's fun to see in Pokemon Stadium, by the way. Very fun.

Holy crap! That's awesome! Yeah, I'm not sure if this is special, but her dragonite knew transform.

Missingno is quite the enigma in the Pokemon world. Anyone know why it appears the way it does? In that weird random L shape?

Someone70 :

MISSINGNO. actually has 5 forms, believe it or not.


The first form is in the L shape that it is most common for. This sprite appears through the old man glitch if the character in the third, fifth, or seventh slot of the player's name is the end-name marker, G, H, J, M, S, T, :, ], a, b, c, m, o, p, or v.

 


The second is actually the unidentified GHOST. Which I guess sort of makes sense. This sprite appears through the old man glitch of the character in the third, fifth, or seventh slot of the player's chosen name is "y"




The fossil of Kabutops is the third form. It appears through the old man glitch and if the character has the letters w in the third, fifth or seventh spot.




...and Areodactyl Same thing. Letter must be X.




The final form is only for Pokemon Yellow, where it uses a messed up sprite of a Pikachu, and thus appears yellow (if on a Gameboy color of course). It can cause different glitches Red and Blue cannot. These once again are rather harmless and don't mess up your actual game, and can be fixed looking at a Pokemon's status in the menu. Obviously the only way to get this form is to trade MISSINGNO in or add it in some other form that isn't the Old man glitch since it was removed in Yellow.






As you can see, the L shape is the most popular because it uses the most characters.

Oh yeah, I saw that. My sisters MISSINGNO appeared in the unidentified ghost form.

Removed.

Here is a fun fact you supposedly will not be able to get missing no in the next full Pokemon game. Why I am not sure but I guess that it is a good idea to somebody.

Yeah! First replies in a while! Anyway, the thing is it depends on the form it appears in. That's happened with many people, wanting to restart the game to get a different form, but if you get either one of the skeleton fossil ones, it wont appear. Or so I've heard.