From Glitch City Laboratories
|Please note that this glitch only exists in the Japanese versions of the game, or is otherwise a glitch from a Pokémon game which was only released in Japan.
The Slow Start glitch is a glitch only existing within the Japanese versions of Pokémon Diamond and Pearl. In these versions, if an attack is used to get rid of the ability Slow Start such as Skill Swap or Worry Seed, the Pokémon initially having the Slow Start ability loses this ability but still must wait 5 turns until the Attack and Speed drops end.
Newer localizations of Pokémon Diamond and Pearl patched this glitch by associating the Attack and Speed drops as integral to the Slow Start ability itself; previously the Slow Start ability functioned only by triggering the effect.