I have to preface today's tip with a serious warning: Do not try this at home! Seriously, doing this glitch can corrupt your save game, costing you hours upon hours of work, or worse. I'm doing it here so you don't have to.

To find the mysterious Glitch Pokémon MissingNo (or possibly 'M), perform the following steps. You'll need to be pretty far into the game (at least far as Cinnabar Island)

1. Start in Viridian City. Find the guy that offered to teach you how to catch Pokémon

2. Take him up on his offer and watch the show

3. When he's done, immediately Fly to Cinnabar Island (yes, I know the picture says Fuchsia City, don't worry about that)

4. And begin Surfing up and down on the thin strip of terrain where the water and the land meet (remember, even encountering this thing can cause some game hiccups, don't do it)

5. Eventually a battle will start, and the Pokémon is a glitchy mass of pixels with a 'M in its name or MissingNO. It's level 0, so it shouldn't be much of a challenge to capture or defeat (extra warning catching it causes lots of glitchy and unpredictable behavior, don't do it)

6. As an added bit of fun, if you fought 'M, like I did here, the sixth item in your inventory will have inexplicably multiplied to an amount so big, that the numbers glitch out trying to display it (around 128)

If MissingNo or 'M is caught, glitches will happen (in fact, just encountering 'M will make weird things happen). But, like I said, this can have disastrous effects on your saved game, whether you save it or not.