Yeah I think I remember that too. In gen 1 there's the "mew glitch" which relies on taking very specific actions so the list of possible wild Pokémon data does not get properly set when transitioning to a new zone.

It's plausible that it could affect fishing too, especially since many of those locations presumably don't have encounter tables, as they're not intended to have wild Pokémon in them.

There were tons of supposed ways to get mew that were total bull. It's kind of fascinating how that info spread across the world's playgrounds

I never knew it to catch Mew, I just remember doing it as a kid for the lolz

The old rod can only catch magikarps. That why he only fished magikarps.

In gen 1 if you loaded an invalid Pokemon you'd get a glitch Pokemon made from garbage code the game reads from where it shouldn't, or MISSINGNO. depending on its Pokedex number. Gen 1 is held together with chewing gum and prayers, and there's very little in the way of error handling. Later games would handle this by defaulting to a giant black and white question mark called ????? that would usually crash the game if you battled with it and could normally only be found using a GameShark.