Did that really happen or is that what your uncle who works at Nintendo told you!
I assume this is a reference to the wonderful Twine horror game "The Uncle Who Works for Nintendo." Just in case you are curious about reality:
It actually happened. I don't remember how I caught MissingNo, I remember it was hard, but I found some way to do so (Maybe I used a Masterball? Maybe I used a status condition?). After I did, its icon in my party was a person, in battle, multiple pokemon had their sprites flipped, there were graphical glitches on the overworld, and when I went to the safari zone, when I tried to leave, the glitches made it impossible to leave the fenced area outside the Safari Zone in Fuschia City.
Of course, as this very game states, this all sounds just like a rumor. I understand if you don't believe me.