I got Missingno in Blue using the Old Man Glitch. I wasn't even aware I'd get Missingno. I knew there was something glitchy about that old man, though, I just wasn't sure what.
I'm going to use Missingno. It's not harmful to my game, is it?
It just seems to flips (horizontally) the sprites in battle and slightly mess up sprites when I attack it, but the game handles it.