When talking to the old man he shows you how to catch a Pokémon so your id becomes Old Man and resets when you enter another area. But if you fly that data doesn't reset so your still Old Man and when you surf on the coast of cinnibar you technically fight the old man.

The Yellow version of Missingno. is way more unstable than the R/B variants. First off, Yellow Missingno., 90% of the time, will crash your game (I didn't say corrupt, just crash). The crashes are mostly "Missingno is trying to learn" over and over again, unkown opcodes (I hate those), just a crash screen, or a jumbled mess of glitchy music.

I've never seen a Yellow version of Missingno (the glitch block ones, not the fossil & ghost forms) that actually work. Sure, the YouTube user MissingnoXpert tried it after about 14 times and it worked.
If you are going to try to catch Missingno, note that it is a pretty unstable mess, and is actually one of the most unstable glitch Pokemon in Yellow.

So, whatever you do, don't try to catch the glitch-block Missingno.s, as it is pretty much a waste of time.
There's a lot of information on this site here about other glitches: http://glitchcity.info/wiki/
And Bulbapedia has some interesting articles about glitches as well (I wouldn't trust ALL of the info, though. Some of the info about glitches aren't true): 
http://bulbapedia.bulbagarden.net/wiki/List_of_glitches_in_Generation_I
http://bulbapedia.bulbagarden.net/wiki/List_of_glitch_Pok%C3%A9mon
http://bulbapedia.bulbagarden.net/wiki/List_of_glitch_moves
http://bulbapedia.bulbagarden.net/wiki/List_of_glitch_types

I have caught Missingno it is not hard but it can cause problems with the game.

Isn't Missingno just a glitch, and isn't that the reason you can't catch it.

I din´t cathched missingno because I dare not

He is able to be caught its just that its a different glitch and has a different effect when you catch him.