Pokemon Red and Blue are home to some of the craziest and most confusing glitches in the entirety of the Pokemon games. Arguably the most popular among these glitches is the Missingno. Glitch. I won't tell you how to do it, because there are already probably thousands of sources that can tell you how.
Basically, it is a glitch that summons Pokemon over Level 100, which, without glitches, is impossible. In most cases, it summons the infamous Missingno., which has a more specific name of random crap M random crap.
My theory on why the glitch happens is that the original Gameboy Pokemon Red had more than 151 slots for Pokemon, and GameFreak just filled them with random glitched ones. When you talk to the old man in Viridian, it does something to the coding and sets the edge of Cinnabar as the location to get a glitched one.
Comment below if you agree or disagree. Also comment if you have... well, a comment.