From Glitch City Laboratories
A Castform is under the effect of the simultaneous weather glitch.
The "simultaneous weather glitch" or often the "Acid Rain glitch" refers to one in Pokémon Diamond, Pearl and Pokémon Platinum which involves the initialization of multiple Weather Effects on the same turn, or arguably the initialisation of an undefined glitch Weather Effect.
When the simultaneous weather glitch is activated the game will cause all of the Weather Effects below the current Weather Effect's priority to happen all at once in the same turn; this also includes Trick Room, Gravity and Uproar which the game technically considers to be the 'top priority weather effects'.
An unusual fact about this glitch is that as long as the player's Pokémon does not have an ability which makes the side effects of the Weather negligible such as Air Lock or Sand Veil all of the normal Weather Effects will deal damage to the player's Pokémon, this even includes the ones which normally do not harm any Pokémon through normal gameplay, such as Rain.
In-game effects which can be used for performing this glitch
- Trick Room - Trick Room is a first priority in-game effect, so the game should display dialogue for all of the in-game effects before it in its priority; those are Gravity, Uproar, Fog, Hail, Sunshine, Sandstorm and Rain.
- Gravity - Gravity is a second priority in-game effect, hence the game should display dialogue for the following; Uproar, Fog, Hail, Sunshine, Sandstorm and Rain.
- Uproar - Uproar is a third priority in-game effect, hence the game should display dialogue for the following: Fog, Hail, Sunshine. Sandstorm and Rain.
- Fog - Fog is a fourth priority in-game effect, hence the game should display dialogue for the following: Hail, Sunshine, Sandstorm and Rain.
- Hail - Hail is a fifth priority in-game effect, hence the game should display dialogue for the following: Sunshine, Sandstorm and Rain.
- Sunshine - Sunshine is a sixth priority in-game effect, hence the game should only display dialogue for Sandstorm and Rain.
- Sandstorm - Sandstorm is a seventh priority in-game effect, hence the game should only diplay dialogue for Rain.
Rain cannot be used to activate the simultaneous weather glitch because it is an eighth priority in-game effect; this is the last valid Weather Effect, therefore the game would set the Weather to normal.
Castform, Cherrim and the Simultaneous Weather Glitch
Presuming that the player activated the glitch with a Weather Effect which has a priority that is above that of Sunshine, such as Trick Room or Hail, the weather effects will alternate between Sunshine and Rain. Since Castform and Cherrim transform into their Sunlight Forme and Positive Forme when the Sun is in effect, Castform or Cherrim would then transform into either their Rain Forme and Negative Forme respectively for when the Rain is in effect; Castform and Cherrim would continuously switch between both of these formes in an endless loop. As a result of this, the player is forced to switch his or her Pokémon game off.
- A Pokémon with any in-game Weather Effect altering Ability or move which is not Drizzle or Rain Dance because Rain is the lowest priority Weather Effect. The player can also use moves such as Trick Room, Gravity and Uproar as the effects of these moves are technically considered to be Weather effects.
- A Pokémon with the move Pursuit.
- The opponent Pokémon must either be on a small amount of HP or have relatively low Defense and Special Defense as the player has to try and defeat an opponent Pokémon with one attack by using Pursuit.
- Enter a link battle with any opponent.
- On the first turn, set up any Weather Effect except Rain. Abomasnow, Groudon and Hippowdon are ideal because their abilities can set up either Hail, Sunlight or a Sandstorm respectively.
- Switch to a Pokémon with the move Pursuit, ideally the opponent should switch to a Pokémon with relatively low HP, Defense or Special Defense so that the player's Pokémon can defeat the opponent's Pokémon in one move by using Pursuit.
- The opponent should attempt to switch his or her Pokémon but the player should use Pursuit and defeat it in one hit.
- When the opponent sends out his or her next Pokémon the simultaneous weather glitch should activate and Weather Effects for all those below the priority of the initial Weather Effect should happen in the same turn.
- Mimic glitch
- Evolve Trade Evolution Pokémon Without Trading
-  - A Youtube video of a player performing the simulataneous weather glitch. He or she has decided to use the Sandstorm Weather Effect.