trust me it happened and i have all the pokemon
Oh, well I have heard of that MissingNo type of Pokemon, but it is really risky. My Red version save file was ruined because of it, and I didn't want to use it.
But..... if you really want Mew, then do this...
1. Activate a code like, all Pokemon in the wild are, for example, Sunflora. Then, if you want to go where I found it, go to Five Island, and go north.
2. There you should Surf, and eventually, a weird Pokemon will appear.
3. Catch it, and raise it. I believe after it gains a level, it will evolve into Mew.
4. If that doesn't work, then if you have a Ruby or Sapphire game, you can try it on that.
This trick is very risky, and I will not take responsibility for your use of this trick.
Get access to the 7th gym by going into the burnt out mansion, go around and you can drop through, then you find a key around there.
Sorry haven't got exact details