When you do the Missingno/Duplicate Items cheat you may think it doesn't mess up you game but it does screw your Hall of Fame/Pokémon League on the P.C.But this is good because when you have beaten the Elite 4 there is a way that you can see all 151 Pokémon, including Mew(well it's obvious if i said all 151 isn't it).Go to any P.C(starting at Viridian City is good) and go to the Pokémon League on the P.C. It will be all weird like the names and lvls of the Pokémon and it will have pictures of Pokémon that you dont even have. If the 3rd Pokémon looks like Missingno, but in a perfect square than it will work, but if it isn't than beat the Elite 4 again so that you have two lists of Pokémon.All your list except the 1st two Pokémon will stay the same each time that you go to the list. Now keep going to the list and just go to the 2nd Pokémon because after a few times of looking at the Missingno Pokémon than it will freeze. Now all the Pokémon that you see will be recorded in your Pokédex but only as ones that you have seen not caught.I am only saying this because i had a Mew the 1st time i did it! I took a picture of him and put it on the computer but i found out that you cant send pictures in on here. The 2nd Pokémon i had was an Electabuzz so I kept on doing it and i saw every Pokémon.Another thing that happens is that the music playing(the Poké Center music)will change each time aswell, i got some pretty cool music a few times and sometimes it has some Pokémon's souns(the sounds that identify them)playing with the music so it sounded pretty cool(especially Jynx's).NOTE:Do not go to the 3rd Pokémon(the one that looks like Missingno)because you game will freeze so you will have to reset you game(Start Select A & B all at once if you didn't know)and all the Pokémon that you have seen will be lost.