|This article is incomplete. Please feel free to add any missing information about the subject. It is missing: Index number.|
Glitch encounter systems are values of the trainer/wild address (0xD057 in Pokémon Red and Blue and 0xD056 in Pokémon Yellow) that were not deliberately meant to be used. There are only 3 unique values, the other ones are copies of 2 of these (03 and 04). All of these are variants of trainer battle. All of these do not correctly register fainted Pokémon. When all glitch encounter (and normal trainer battle mode) systems are combined with Safari mode, the game allows you to run. In Yellow, the modes are the same.
00Allows overworld items to be used. Battle related items, are not allowed. This value is in fact used for when the player is outside of battle, but serves as a glitch encounter system in battle. When forced with Safari mode, the game allows you to use bait and rocks.
03 and 04 (onward)Balls are blocked, but first wobble. Their animation plays at a wrong place and your own Pokémon is removed. 04 is essentially the same, except the animation plays at the right place and the correct Pokémon sprite is removed. Just like normal trainer mode, Teleport will fail.
When forced outside of battleIt is possible to use - (Generation I move) corruption to obtain invalid D057 values outside of battle. This can lead to a glitchy Poké Ball throwing animation which is capable of Tile Chaos in Pokémon Red and Blue, and other effects which vary based on the version.
In (at least) Pokémon Yellow, you can get a Glitch City. In Pokémon Red you can get a glitchy mess of tiles and animations. In Pokémon Green (v1.0) you can get a lock-up, a Trainer blocking the ball, or activate a'broken start menu' effect, where the menu is invisible when you press start until you save and reset. In Pokémon Green (v1.1) you can get a glitchy mess like in Red, but music may play. In Japanese Blue you can have the game lock up or both warp you somewhere and break the start menu. In Japanese Yellow (v1.0) you can get animated water drops on the screen with the 'follow me' music playing. In Japanese Yellow (v1.1 and v1.2) you can get a glitchy mess of tiles. The effect is probably the same between v1.1 and v1.2.
(text from above list taken from ChickasaurusGL's YouTube video)