In every Pokemon game, there are dark caves that require the move "Flash" for you to be able to see inside them. However, have you ever tried getting through them without it?
I discovered this trick on Pokemon Gold and have since tried it on every Pokemon game I own. It works on them all, even Gen 1 games.
You must be in a dark cave or anywhere you should use Flash. Walk around, fight Pokemon in the dark, bump into trainers, and keep wandering in the dark. Eventually, you will find a ladder that isn't on the map. Go down it.
The screen will go black as you hear the 'going down stairs' nnoise, but it will play four times, as if you've just gone down four floors.
A text box will appear on the black screen, simply saying "Where do you think you are?" The music will be distorted for a few seconds as the screen glitches up. Sometimes you might hear a 'thud' noise as if you walked into a wall or a distorted Pokemon cry that sounds like a scream.
After that, you'll be able to see. In the older generations of Pokemon, it simply looked like you'd found a new area that wasn't on the map. In newer generations, you can tell something is wrong because everything is in black and white.
You'll be in a big, empty room. The four walls all have something written on them - usually your player name and the time or the names of your Pokemon. You can walk through one of the walls, but which one depends on the game. If I remember correctly, it's the north wall in Diamond and the left one in Yellow...so on, so forth.
When you go through the wall, you'll be back in the overworld outside the cave. Everything will still be in black and white. The music will be slower and lower; occasionally, it jumps, too. Trying to talk to people is useless; you can't interact with them any more.
In Pokemon Yellow, the sprite of the Pikachu that follows you around becomes that of a Ghost from Lavender Town. If you check your Pokemon, they will all be there - same stats, nicknames, and moves - but all their sprites have changed into that of the Ghost.
If you go into the grass and find Wild Pokemon, they will amost immediately flee from you.
Go back into the dark cave you came out of. This is now the only place you can fight Wild Pokemon and other trainers, so long as you don't use Flash.
I discovered recently that if you play the game like this for long enough, you will eventually see a trainer that uses the main character's sprite in that one dark cave. They stand out, for they are in color when everything else is not.
However, if you try to talk to them, a text box pops up saying "Where do you think you are?" The screen goes black and you hear the distorted music, thud, or scream from before...