Someone want to give a little explanation of missingno? Not a big Pokefan

A glitch that I never forget for some reason was in Beyond Good and Evil I was in some sewers or something and you had to jump down in these openings to hide while the guards passed and one of them I jumped into glitched and I fell through the floor into eternity. I think a lot of games have that kinda glitch. They're rare to hit and don't usually happen more than once. But I've heard of others. It was interesting to watch