Yeah, the Missingno I caught was a Tetris block. Of course, he had several forms... sometimes his name can be a jumble of shapes and the letter "M". I've never seen a ghost Missingno though. I don't remember him screwing with my game that badly; the battle animations got screwy but that was it.
One fake code that still gets me is in Super Mario 64... you walked along the wall by the Cool Cool Mountain painting to make the ghost laugh (and move to behind the wall in that hallway), and you were supposed to jump over the door somehow, and the ghost would tell you how to get Luigi. After about six years, I finally managed to get into the black void with the ghost, and it really pissed me off that he didn't talk to me!
(And by the way, you can't jump over the door... it takes a bit of cleverness to get back with that ghost.)
However, I invented the *legitimate* "pseudo-first-person Mario 64" code. So, if anyone wants me to share...