Bug: Go to Bill's house. Before you enter the door, watch quickly at the back of his house. You'll see a mountain without the usual border.
Bug: For this glitch, advance through the game until you get the FLY HM and get a Pokemon that can learn how to fly. Also, do not fight the trainer in the Cape that only has a Slowpoke, nor the trainer that is looking up outside the Underground Path (right entrance). Go to this entrance of the Underground Path and face down. Press down and immediately press Start. You'll be in the trainer's sighting, but he won't react to this. Go to Pokemon, choose the Pokemon that knows how to Fly and fly to Cerulean City. The trainer will now notice you, but you'll fly away. In Cerulean City, the Start menu won't work. Now go to the Cape and let the Slowpoke trainer see you and walk to you. Defeat his Slowpoke and the Start menu will work again. Fly to Lavender town, and go to the route that heads to the left. When you enter the route, the Start button will pop up for no reason. Press B, and a wild Pokemon animation will show up. From nowhere, a Mew, an impossible-to-get Pokemon, will want to fight with you.