Pokemon Red Version

i dont know what a fossil missingno is but my advice would be NOT TO TRADE if your missingno was level 0 when you caught it, there is a number of cheats for the game that work with missingno and level 0 missingno has been known to corrupt save files and in the worst cases corrupt game cartridges, if its level anything other than 0 than you should be fine..
THATS MY ADVICE, i dont want to be held responsible if your game breaks

I've heard that catching Missingno. isn't different from seeing him. Once you see it, the game is affected by it. But either way, just seeing it can cause problems and even delete your game.

Don't catch or trade a Missingno. This was a pokemon that was never supposed to be found in this game and doing so might exploit some files and cause your game files to become corrupt. My advice would be to start over....and don't go looking for the damn thing.

If you've seen Missingno, then the damage is done. Making a new file won't do anything. If you caught it, just release it.

Everybody just keeps telling you not to catch it, huh?
Well, I for one can afford $10 for a used copy of the game, so I'm not worried about it messing up AND SO...
-----Everything You Want To Know About The Mysterious Pokemon "MISSINGNO."-----
- Note: Short version at bottom. -
Missingno is Bird/Normal type. (not Flying type, Bird was a beta type not used for any other Pokemon)
Missingno is theoretically weak against Fighting type attacks, and is immune to Ghost type attacks, in reality however, I believe it has no weaknesses or resistances.
Missingno. appears as a baby Rhydon (the image used in the substitute attack) when viewed in Pokemon Stadium.
Missingno. appears as a Ditto when viewed in Pokemon Stadium 2, and if the game is then saved from Stadium 2, the Pokemon will be a Ditto permanently.
Missingno. has no adverse effects on your game other than the following: Hall of Fame data will be corrupted, some graphics may temporarily glitch and otherwise be altered, and your game may freeze.
Missingno. has 3 moves: Water Gun, Water Gun (again) and Sky Attack.
Missingno. can learn TMs 01-03, 05, 06, 09-11, 13, 14, 17, 19, 20, 25-27, 29, 30, 43-45, 49, 50, and HMs 01 (Cut) and 02 (Fly).
- Note: The TM/HM compatibility list above may differ depending on which Pokemon you encountered before battling Missingno.
- Note: Missingno. can also be caught using the alias: 'M. - Having said that...
'M - unlike Missingno. - actually learns a move. "Pound". At level 138.
'M - unlike Missingno. - actually even evolves! ...Into Kangaskhan at level 1, Clefairy at level 128, and Clefable if you use a Moon Stone on said Clefairy.
- Note: This means that you could have a level 1 Kangaskhan with moves like Water Gun and Fly.
- Note: 'M can only evolve into Clefairy if you caught it at a level exceeding 100.
- Note: 'M can also appear as a Charizard (commonly called "Charizard 'M"), which is much more dangerous a glitch than Missingno.
Charizard 'M can make all of your Pokemon turn into Charizard 'M as well (they will keep their own types, however).
Charizard 'M can also change your name to "Z", interrupt battle with a new battle, or turn your entire party into Bulbasaurs.
Charizard 'M's "cry" is loud static, and can sometimes cause the game's BGM (background music) to stop playing.
Charizard 'M isn't even counted by the game as a Pokemon in some cases, for example, it cannot be healed at a Pokemon Center.
Charizard 'M is most commonly found as a Charizard (thus the name), but can sometimes be seen as other Pokemon, like Ditto.
Charizard 'M can be created by a trading error if the link cable has dirty connectors or is otherwise damaged.

SHORT VERSION: Missingno. WILL NOT damage your game AT ALL, however, any the Pokemon called 'M is very likely to if it doesn't look like Missingno. (backwards L shape).