Okay this cheat is to get more than one of a certain item. Like a master ball for example, you can get more than one with this cheat.
Step 1: Go to Veridian City
Step 2: Talk to the Old Man that didn't have his coffee so he was cranky
Step 3: Let him demonstrate to you how to catch a Pokemon
Step 4: Fly to the place where you get your 7th Gym Badge and go to the very left of the island and prepare to surf
Step 5: You see the darkish-colored mud in front of you? Surf along that and eventually you will run into MissingNo.
*Note: You must Have all the TMs listed to do this
-Ruby the Eevee