How to catch Missing but in Pokemon Red and Blue

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1 Name your character "ASH" without quotes. Capturing Missing But requires certain letters in the name, and ASH is the easiest option.

2 Travel to Viridian City and speak to the old man in the northern part of the city. When he asks you if you are in a hurry, say “No”.

3 After seeing the demonstration, immediately fly to Cinnabar Island. You can also fly to Fuchsia City.

4 Sail to the east coast of the island or, if you flew to Fuchsia, sail south and then west until you reach the Seafoam Islands.

5 When you reach Cinnabar Island, start sailing along the coast.

6 After a while you will meet MissingNo. You can also see M, which is also good.

If you named your character differently, then you will meet a Pokemon with a similar name, which will have a level above 100. For example, Mason may meet Magnemite level 174. If your character's name is not ASH, there is a chance the Pokémon will appear as a ghost, such as a Kabutops skeleton or Aerodactyl skeleton.
Using this method, you will receive about 128 pieces of the item, which is located in the sixth slot of your backpack. This is because a Pokedex value of 128 is used, which determines whether you have seen a Pokemon or not. This value is displayed as the amount of that item. You don't have to catch Missing for that to happen, but you just need to meet him. You can win, catch or run away from him.
MissingBut has very low basic characteristics, but the highest attack of all Pokemon presented in Pokemon Red and Blue.

MissingBut has a 37% chance of being caught. If the MissingBo you meet has level 0, do not catch it, unless, of course, you have free space in the group. If, after being caught, the Pokémon is sent straight to the computer, Level 0 MissingNo will freeze the game after you try to get it. You can fix this only by letting go of it. Missing But any other level will interact with the computer, and you can get them out of it.
You can transfer MissingNo to the second generation games (Pokemon Gold / Silver / Crystal), but after that he will become the second evolution Pokemon, most often Tyroug.
It is highly recommended not to use MissingNo in Pokémon Stadium on Nintendo 64, as MissingNo will almost certainly crash the game, after which you will have to turn off the console, turn it on again and restart the last game.