1st Gen A found Missingno with Mew glitch

[Ladymeganium]

This is Ladymeganium and I found Missingno using the mew glitch.

I did this by telporting away from the trainer in the grass on route 24.

Then on route 11 there is a gambler with a level 18 vulpix and if you defeat that vulpix and telport back. Then go to nugget bridge a wild missingno would appear.

I am not sure if it gets the same effect as the old man glitch with duplicating the sixth item in your slot but I thought this would be something interesting to share.

 

[Ladymeganium]

I discovered that if you do use the mew glitch to find missingno it does duplicate the sixth item in your slot

 

[mew_nani]

Believe it or not you can actually encounter almost every available Pokemon in Gen I with the Mew Glitch (and even a few that aren't!) The reason you encountered a Missingno. was because the last trainer you battled had a special stat that corresponds to one of the index numbers Missingno. resides in. There are 190 slots in Pokemon R/B/Y that have "valid" Pokemon in them, and they're listed in the order they were programed into the game; thus Rhydon is at the very top of the list at slot 01. Many of the slots had Pokemon removed in development however, and all 39 of these were filled with Missingno. as a placeholder. Here's a handy little list of the index numbers for you: https://bulbapedia.bulbagarden.net/wiki/List_of_Pok%C3%A9mon_by_index_number_(Generation_I)

You can actually control what Pokemon you summon through the Mew Glitch by performing an extended variant with a Ditto. The glitch summons a Pokemon based on the special stat of the last Pokemon you encountered; by encountering a Ditto before returning to Nugget Bridge (or the place you first initiated the Mew Glitch) and allowing it to Transform into a Pokemon you own, you can freely alter what gets summoned by just sending out a Pokemon with a specific special stat. You can even get glitch Pokemon this way, though you can't get all of them because once you reach a special stat of 200 you start summoning glitch Trainers instead. For added fun you can also modify their level via modifying the Ditto's attack stat, and by growling at it 6 times you can get a level 1 Pokemon, which you can level up to level 100 if you give it experience, but not enough to level it up. Just remember to either make the Ditto faint or run away, and be warned; many of the glitch Pokemon have Pokedex entries that the game won't really exit out of, and you will need to either see or catch certain Pokemon in order to bypass this. Not all of them do though so it's ok.

As for Missingno's item duplication abilities, those are constant no matter how you encounter it. Each Pokmon has a byte attached to its Pokedex entry; this byte is at 0 if neither seen nor caught, 128 if seen but not caught, and 255 if the Pokemon is both seen and caught. For Missingno this byte happens to be the 6th item in your bag; this item will be duplicated each time Missingno is seen or caught. You can actually do some really cool stuff with this item duplication glitch beyond getting a ton of Master Balls and other rare items but it's a bit complicated to set up.