This form appears through the old man glitch if the character in the third, fifth, or seventh slot of the player's name is the end-name marker, G, H, J, M, S, T, :, ], a, b, c, m, o, p, or v.
This form appears through the Mew glitch with a Special stat of 31, 32, 50, 52, 56, 61, 62, 63, 67, 68, 69, 79, 80, 81, 86, 87, 94, 95, 115, 121, 122, 127, 134, 135, 137, 140, 146, 156, 159, 160, 161, 162, 172, 174, 175 or 181.
This form can be obtained through the Johto guard glitch if the traded Pokémon is Scizor, Shuckle, Heracross, Sneasel, Teddiursa, Ursaring, Slugma, Magcargo, Swinub, Piloswine, Corsola, Remoraid, Octillery, Delibird, Mantine, Skarmory, Houndour, Houndoom, Kingdra, Phanpy, Donphan, Porygon2, Stantler, Smeargle, Tyrogue, Hitmontop, Smoochum, Elekid, Magby, Miltank, Blissey, Raikou, Entei, Suicune, Larvitar, or Ho-Oh.
Missingno.'s appearance in-battle allows the item duplication glitch,
which increases the number of items in the 6th Bag slot by 128 unless
the number of items in the slot already exceeds 128. Encountering
Missingno. has been known to interfere with the save game data in
various ways, such as adversely affecting the Hall of Fame saved data
and (if its stats are viewed) messing with some graphics until the
stats of a non-glitch Pokémon are viewed. When a Trainer battles with
Missingno. in their party, both the Trainer sprite and their Pokémon's
sprites will be scrambled, and all other sprites will be reversed.
Viewing the stats screen of a non-glitch Pokémon will remove the
effects. In Pokémon Yellow, capturing Missingno. will cause multiple
player sprites to walk around the screen.
Misssingno is a threat to all the games it appears in like most glitches are. So it is not recommended to try and get it in your ed/Blue/Yellow game unless you want it to crash or you don't want to ever see that game again