In all versions of Pokemon Platinum, HeartGold and SoulSilver (Generation IV) games, a glitch can occur when any weather effect can potentially cause all four weather effects (Sunny, Rainy, Sandstorm, and Hail; the foggy condition not appearing in the glitch) to trigger simultaneously and cause damage to all Pokemon. This glitch is commonly known as the “Acid Rain” glitch, because it damages Pokemon four times, regardless of their immunities (for example, abilities like Pikachu’s Static and Empoleon’s Torrent will cause damage four times, rather than prevent it, and Pokemon that have immunity because of their type will not receive the benefit of immunity). However if a Pokemon is immune to both Hail and Sandstorm due to typing (a Ground/Ice Pokemon like Swinub, for example), then that Pokemon will be immune to the effects of the “acid rain” glitch. When the Pokemon take damage from the weather, the game will cite the reason to their ability (so a Pikachu taking damage from the weather will cause the game to state “Pikachu is hurt by its Static!”.
Abilities that cause Pokemon to heal instead such as Rain Dish (heals during rain) or Ice Body (heals in hail) will receive a massive boost in combat, as their ability will cause them to heal completely, then forces them to take a small amount of damage from the ‘damaging weather’. Abilities that heal in some weather and hurt in others will activate, causing the Pokemon to regain some of its lost HP, then damages them.
Pokemon moves that are affected by weather are also affected by the Acid Rain glitch. Solarbeam for example will not have to charge because of the sunny condition, but will also only do half as much damage as it normally would, due to the other weather effects in play. Meanwhile both the accuracy of Thunder and Blizzard will increased to 100%, and will never miss, as both rain and hail are in effect, which raises the accuracy of both attacks respectively.
Apart from the annoyance of taking constant damage, the only inherent danger of battling with Acid Rain in effect is if Castform or Cherrim are brought onto the field. Both Pokemon have abilities which cause them to transform based on however, and because of the fact that all four weather conditions are in effect, they will begin to cycle through their transformations– locking the battle up until the game is reset. Strangely enough Castform will only change into its Sun and Rain forms as it can only recognise these two as the only weather effects in play (even though it has a form for fail). This can be beneficial to either yourself, or your opposition, depending on the Pokemon that are in play – Especially when you consider that Swinub is one of a select few Pokemon that is adversely (neither positively) affected by such a glitch.
The activation of this glitch is hit and miss, because it requires perfect timing. To activate it, the attacking Pokemon must use Pursuit against the defending Pokemon as they switch, and cause them to faint. All of this must be done while a weather effect of any type is present, on the exception of the rainy weather condition, which will not cause the glitch to trigger. This then leads to all weather effects coming into play and continuing to take effect until the battle ends. When battling against a Diamond and Pearl game, it is possible to cause Acid Rain to come into effect, even though the glitch does not occur in Diamond and Pearl, so long as the user of Pursuit is playing Platinum, Heart Gold, or Soul Silver. As of now there is no known reason why the Acid Rain glitch occurs.