Question to whoever wants to answer it: What do you consider to be the difference, if any, between a glitch and a bug? What about a programmer oversight?
@Felthry It's an interesting question. While I do mostly use them interchangeably, thinking about the split more: I consider MissingNo a glitch, but the component elements (the Old Man trick and Cinnabar Island not being programmed properly) more as bugs. The game is working "as programmed" with the bugs, while you are very clearly not meant to encounter this thing or have it duplicate your 6th item slot?
@Dex Honestly, the duplicating the sixth item slot could be categorized as a programmer oversight or even working-as-intended, since it's just the game setting the pokedex seen flag for missingno, which just so happens to be the high bit of the quantity of the sixth item. Since you aren't supposed to be able to encounter a pokemon with pokedex number outside the 1-151 range, they didn't consider what would happen if the game tried to set seen flags for them.
@Dex I know that catching it triggers the seen flag again if you've tossed some items to get the number low enough (hence the standard trick of see it, toss two items, catch it to get 255), but the caught flag is somewhere else I think
@Felthry it hits the same location, IIRC, it just gives 255 rather than 128 - but it can behave weirdly if you've already used some of the items IIRC, it's been a while
It can be useful for obtaining an expanded items pack though
I also think it displays a glitchy Pokédex entry if you haven't seen Marowak or Ninetails or Rapidash or something