Glitch: Old Man
This glitch requires you to have the Badges allowing the use of Fly and Surf, and Pokemon that can use these moves. You must also be able to reach Fuschia City or Cinnabar Island. First, head to Viridian City and speak with the old man near Route 2 who will teach you to catch Pokemon; view his tutorial. After doing so, Fly to Fuschia or Cinnabar and Surf to the Seafoam Islands. Once there, Surf on the coastline of the islands. As you Surf, you may encounter a number of odd Pokemon. These Pokemon will differ, depending on the name you used as that of your Trainer. For example, the name "DANIEL" can make Snorlax or Electrode appear, among others. However, a common characteristic of them is that they are often above Level 100 and thus very powerful: if you manage to catch one somehow, you essentially have a very powerful Pokemon, and can even level it up further, as high as Level 255. (Note that some Pokemon may evolve into glitch Pokemon because of this.) That is the first benefit of this simple glitch. Most people, regardless of name, should be able to encounter one of two Pokemon: Missingno. or 'M, both of which are glitch Pokemon. These two have special effects on the item in the sixth slot of your item bag, but that effect depends on whether you catch or defeat the Pokemon. If you defeat the Pokemon (or otherwise just do not catch it), the item in your Bag's sixth slot will be changed so that you have 128 of it; if you catch it, you will have 255 of it. (This is assuming you use an item that you can have multiple ones of.) This is very useful for mass-cloning valuable items, such as Master Balls, Rare Candies, or PP Maxes, or to clone TMs to sell for obscene amounts of money.