Missingno. has actually been proven to be relatively harmless to save files, with the only real damage inflicted by the thing being the Hall of Fame being permanently corrupted. Other glitch Pokemon similar in appearance and function to Missingno. such as the glitch Pokemon .4 are more dangerous however - .4 has been confirmed to be able to erase save files using one of its glitch attacks. Missingno. itself however is harmless, and can be used and messed around with pretty safely.
Also, although the item duplication glitch was removed from Pokemon Yellow, Missingno. can still be encountered using a different method, which brings me onto this glitch I messed with several months ago:
The Mew Glitch - Pokemon Yellow
This is exactly what it says it is - this glitch allows you to acquire Mew in Pokemon Yellow without the use of a Gameshark, Action Replay or any other cheating device, nor does it require downloading Mew from an event. The Mew itself, once caught, is perfectly normal, and operates exactly as a regular Mew would.
This earliest this glitch can be performed is just after the Nugget Bridge, on the route leading to Bill's house. Once you've beaten all the trainers on the Bridge as well as the Rocket Grunt, turn left to where you meet the boy who gives you Charmander. South of his location is a trainer who will help in performing the glitch. Save at this point. Another thing of note is that you'll need a Pokemon with Teleport or Fly in your party here - the easiest method for this is to trade for a Teleporting Pokemon or catch yourself an Abra. Now, walk down to where the trainer is, so that you're directly in line with where he is - the instant he appears on the screen, he will challenge you to a battle (Note that this glitch will not work if you've already beaten this guy.) Now, as soon as the trainer appears on-screen, IMMEDIATELY press Start. If you whiff the timing and he challenges you, reset to your last save point - this is why I told you to save earlier. If you timed it right, the menu will appear the moment he appears on-screen. Then, go to your Pokemon menu and select your Teleporting/Flying Pokemon, and use it to head back to the Pokemon Center in Cerulean City (or any city, doesn't really matter, although Cerulean is the closest to the next location you must go for the glitch.) The trainer who would have battled you will get the signature "!" mark over his head and approach the spot where you once were, but the battle will not take place since you're no longer there.
Now, go back to the Nugget Bridge and onto the route to Bill's house. Look for a trainer on the route who has a Slowpoke - it HAS to be this trainer, and if you've already beaten him then you can't do this glitch. Also, you must NOT battle anyone or anything else between Teleporting/Flying back to Cerulean and battling the Trainer with the Slowpoke, so if you get the wrong trainer or are attacked by a wild Pokemon, reset from your last save. Anyway, find the kid with the Slowpoke and beat him, then head back to the Nugget Bridge. You may have noticed that your Start Menu has been completely disabled since Teleporting - this is normal. On your way back to the Nugget Bridge, your Start Menu will suddenly reappear out of nowhere. Close the menu, and you'll immediately be attacked by a wild Mew, around lv.7 or 8. Catch it (it's pretty easy) and congratulations, you have a (somewhat legit) Mew!
Now, Missingno. comes into this due to how the glitch works - the Pokemon that appears when the menu pops up is determined by whatever the last Pokemon you battled was - in this case, the Slowpoke you battled caused Mew to appear. Battling a different Pokemon instead of the kid's Slowpoke will cause a different Pokemon to appear when the menu pops up, and sometimes this Pokemon will be Missingno. or some variant of it. A fiddly glitch to work, but the potential rewards are definitely worth it.
A few things of note:
- If you have beaten the trainer to the left of Nugget Bridge, this glitch CAN still be done - I lied a bit there. This part of the glitch can be done with any trainer who immediately spots you as soon as they appear on the screen. Another example is one of the Gamblers on the route to Celadon City.
- I don't know if this glitch can be performed repeatedly, or if it can be repeated to any real effect. Obviously getting two Mews here is not that easy since you can't battle the Slowpoke kid more than once, although you could mess with it to see what results you can get from it. Beware of .4 however, since it can appear with this glitch and can erase or permanently corrupt saved data if you aren't careful.