Well you didn't know it before hand... So that doesn't count.
It's for sure at the top 10 most legendary glitches ever.
So many rumors have been made about it, but it all comes down to coding errors. It's amazing what the glitch is capable of doing. Like making all your Pokemon pretty much invincible. It was a huge deal for a 8-10 year old kid on the school playground when i was a kid. (Not to mention the graphical mindfuck it was able to do. Scaring a lot of kids who though their game and profile data would break)
Nintendo even referenced it later on in their game lineup. and a lot of impressive fan art have been made. Like what would it look like if Mewtwo and Missingno made a baby... here it is: https://pbs.twimg.com/media/BuNo4LZIMAAPTe5.jpg:large
Here is a picture of Missingno:
Imagine meeting him for the first time.