For no real reason, I started playing Pokémon Blue again. After a couple of days, I remembered the legendary "MissingNo" glitch! This happens when you Fly to Viridan City, speak to the man, let him teach you how to catch pokémon, fly to cinnabar islands and surf along the east coast.
Either a L100+ monster appears, the monster varies according to your name, or MissingNo. If you kill / run from MissingNo, it makes the 6th item in your inventory be cloned 127 times. This means a nugget, which sells for 5k, can be cloned many times! Also, rare candy, which instantly levels your pokemon, can be got instantly. This means you can have a L100 pokemon in about 10 minutes!