Missingno.

For those who don't know missingno is a 1st generation pokémon glitch. Only happening in the red and blue versions, people have used this glitch to spark different theories all over pokémon fandom.

Missingno stands for missing number, which is just an empty slot used up in the games memory. You can encounter this by talking to the old man in Viridian city, this resets your hexadecimal values so you can catch the missingno, go to cinnabar island and surf along the right hand side, right next to the shore. You can duplicate items through this glitch.

Rumors are thing such as because it is a flying type, it may have been an unreleased legendary bird. Or it would evolve into a kangaskhan, ditto, mew or Ryhdon. Other rumors said that the missingno were ways to achieve the second generation pokémon which were out at the time. The missingno glitch is harmless towards your game and cannot break it, but it can make some glitchy things happen especially in the hall of fame.