Trainer escape-Ghost glitch: Bypass bad sprites (4 4 (BF) etc.), catch them/duplicate/mutate items
66Notes: Not to be confused with encountering Fossil/Ghost MissingNo. to duplicate items, you can turn any Pokémon/glitch Pokemon from IDs 1-199 into an actual Pokémon Tower Ghost by performing a death warp at Pokémon Tower (you'll temporarily need to have the Silph Scope, then deposit it in the PC or do a glitch to get rid of it), doing the usual Trainer escape steps, and as usual, manipulating the right Special stat.
While this may seem useless at a glance, the ghosts do set encounter flags (due to an oversight where you can register Pokémon in the Pokédex from unrevealed ghosts); so for instance you can get a lower easier to acquire Special stat to encounter the normally unstable Yellow MissingNo. (such as Special stat 31) which will show the ghost sprite instead of its own, and it will duplicate the sixth item as usual (if it's under x128). Also of note, the palette is of the actual Pokémon/glitch Pokémon and furthermore any glitch screens/palette attribute effects will be applied, so on Game Boy Color the 4 4 Ghost will appear fully black. From here are some additional applications.
If you perform the glitch where you can reveal ghost sprites (but not preventing the text that you can't escape) by viewing the summary screen of a party Pokémon, then it's possible for bad sprite glitch Pokémon like unstable MissingNo. and 4 4 (BF) to apply their corruptions immediately after leaving and returning to the party list. With those previously mentioned two, so far I've only corrupted the music and frozen the game, but I wonder if you can do some more interesting things with other glitch Pokémon.
If you have the expanded inventory, then you can change item 33 to disable the ghost effects (except things like the sprite); because this represents the current map and ghosts only appear in the Pokémon Tower. For instance, you can swap it to the " x " glitch item representing map 0 (Pallet Town). This will let you throw a Master Ball (etc.) to catch the Pokémon; including harder to acquire glitch Pokémon like 4 4 (BF).
Also note while this was performed on Yellow it probably works the same in Red/Blue, so you could maybe get Pokémon like PokéWTrainer this way too.