Secrets would mean like Glitches and things that wouldn't be known to the average player.
This is open for everyone to post and learn.
After all the more you read the more you learn I guess.
Please post videos or VERY detailed text when telling about a glitch or a secret.
This is open to all Pok?mon games on vizzed!!!
Enjoy the glitches and the secrets!
(pictures are acceptable)

Cool!
Let's start with how to catch the notorious Missingno. in Pokémon Red and Blue.
Catching Missingno. depends on having certain letters in your character's name. Try naming your character ASH.
Next, you should talk to the old man in Viridian City.
When it says "Are you in a hurry?" say "No"
After that, fly to Cinnabar Island. Go to the east of the island, and surf right on the coastline, going up or down. After a while, you will encounter Missingno. or simply "M"

When doing this, your item in the 6th slot of your bag will be duplicated - 128 copies of it. This will happen when you encounter Missingno. You don't have to catch it, you can simply kill it or run away from it. 
Missingno. has the highest ATTACK stat of any Pokémon in Red/Blue, but very low base stats. 


Now, here's the well-known Emerald cloning glitch!
You must have access to the Battle Frontier though.

Go to the Battle Tower and go to the PC.
Deposit any Pokémon you want to clone into Lannette's PC. To Clone items, make any Pokémon you deposit into the PC hold that item.
Finish that, then save your game. 
Go to the PC, withdraw the Pokémon you want to clone, and talk to the lady behind the counter directly to your left.
You will be asked if you want to make a challenge - select "Yes". Choose any two eligible Pokémon in your party.
You will then be asked to save your game. Select "Yes" 
The game will processing, so it appears to be frozen (just like anytime you save lol, nothing's wrong)
This is where many people mess up. They turn off the game during that processing. You need to turn the game AFTER the slight freeze. 
When you load your game, the cloned Pokémon and items will be both in your party and your PC (Lannette's PC)

Well this isn't really a secret or glitch but everyone knows of the whole Skill Swap Sablyeye and Shedinja trick. Having something that can't be directly attacked is actually pretty effective.

And here's a very little known secret.

In the Red, Blue and Green game's coding, it's possible to have the Boulderbadge and Cascadebadge as items. These items act like Rock and Bait respectively from the Safari zone, but can be used anywhere in any battle.

Ta da

ignavusd : That second bit sounds pretty cool! Do ya know how to do it, I would love to try it.

Wow this is really geowing!
This is going to be great keep it up!

I remember the good old days when you can clone pokemon and items in Gold.
It went something like this.
1. Equip a valuable item to a pokemon in your party such as a rare candy or masterball and place the pokemon in an PC box, and then switch to another box.
At some point it will ask if you want to save, click yes and turn off the game while it is saving. When you turn the game back on, you will have the pokemon with the item both in your party and in your PC.


Also, if a pokemon is obtained at level 100, you can keep using rare candies and the pokemon will keep leveling up to level 255. If you use a rare candy at level 255, level will reset to 0 and if pokemon gains level by exp, it will reset to 100. This is found in all Generation1 and 2 games.

Well, there are a lot of glitches in the original games. Like the warp glitches for TAS and beat Red in a minute. Or do the Pallet Town glitch or even the Mew glitch for example in Red and Blue. There are a LOT of glitches in these games.

I know of a few glitches. 

In Red/Blue you can use a Pokedoll on the Ghost in the Lavender Town Tower (Cant remember the name) and it works the same as the sliph. scope. 

In Sapphire/Ruby there is a glitch where if you deal a large amount of damage the HP bar will not go down at all. In fact it will stay the same as the Pokemon faints.

There are a few more minor glitches in the Ruby/Sapphire/Emerald series but they are minor and I don't feel like mentioning them right now. XD

steel_attacker : Do you have any links to videos?
they would be really helpful!